The development of the trigeminal jaw adductor musculature in the grass snake Natrix natrix

The ontogenetic development of the jaw adductor musculature in Natrix natrix (L.) is described in detail and related to patterns of ossification in the skull. A comparison with the development of the jaw adductors in the lizard Podarcis sicula reveals some interesting differences of dynamics in the developing head. The problem of the establishment of topological relations of similarity (homology) in developing systems is discussed.
The homologies of the external jaw adductor compartments in lizards and snakes are revised. Early ontogenetic divergence explains the reversal of fibre direction in the anterior portion of the external adductor of snakes, and renders the homologization of that fibre bundle with part of the external adductor in lizards impossible.     

The Limited Role of Nonnative Contacts in the Folding Pathways of a Lattice Protein

Models of protein energetics that neglect interactions between amino acids that are not adjacent in the native state, such as the Gō model, encode or underlie many influential ideas on protein folding. Implicit in this simplification is a crucial assumption that has never been critically evaluated in a broad context: Detailed mechanisms of protein folding are not biased by nonnative contacts, typically argued to be a consequence of sequence design and/or topology. Here we present, using computer simulations of a well-studied lattice heteropolymer model, the first systematic test of this oft-assumed correspondence over the statistically significant range of hundreds of thousands of amino acid sequences that fold to the same native structure. Contrary to previous conjectures, we find a multiplicity of folding mechanisms, suggesting that Gō-like models cannot be justified by considerations of topology alone. Instead, we find that the crucial factor in discriminating among topological pathways is the heterogeneity of native contact energies: The order in which native contacts accumulate is profoundly insensitive to omission of nonnative interactions, provided that native contact heterogeneity is retained. This robustness holds over a surprisingly wide range of folding rates for our designed sequences. Mirroring predictions based on the principle of minimum frustration, fast-folding sequences match their Gō-like counterparts in both topological mechanism and transit times. Less optimized sequences dwell much longer in the unfolded state and/or off-pathway intermediates than do Gō-like models. For dynamics that bridge unfolded and unfolded states, however, even slow folders exhibit topological mechanisms and transit times nearly identical with those of their Gō-like counterparts. Our results do not imply a direct correspondence between folding trajectories of Gō-like models and those of real proteins, but they do help to clarify key topological and energetic assumptions that are commonly used to justify such caricatures.     

Sequences specific for enterovirus detected in spinal cord from patients with motor neurone disease.

OBJECTIVE--To investigate the association of enteroviruses with motor neurone disease, also known as amyotrophic lateral sclerosis. DESIGN--Analysis by enterovirus polymerase chain reaction of wax embedded material from spinal cords taken at necropsy from subjects with motor neurone disease and from age and sex matched controls. SETTING--Specimens were collected in the west of Scotland and in London between 1982 and 1992. RESULTS--Sequences specific for a non-poliovirus type enterovirus were detected in spinal cord tissue from subjects with motor neurone disease. Amplification of a 414 base RNA target sequence in the conserved enterovirus 5'' untranslated region from wax embedded tissue sections was successful in tissue from eight of 11 cases of sporadic motor neurone disease, one of two cases of familial motor neurone disease, and the one case of poliomyelitis, but not in the six matched controls or one case of antecedent poliomyelitis. In addition, sequences were detected in spinal cords from one monkey infected with wild type poliovirus and one monkey infected with polio vaccine. Comparison of sequences from cases of motor neurone disease with sequences of corresponding regions of the 5'' untranslated regions of known picornaviruses showed them to be tightly grouped within the enterovirus genus closely related to coxsackievirus type B but not to polioviruses. Sequences derived from different parts of the spinal cord of the same subjects were identical, but sequences differed between individual subjects. CONCLUSIONS--Conserved enteroviral sequences closely related to coxsackie B virus sequences were detectable in spinal cords from subjects with sporadic motor neurone disease and from one subject with possible familial motor neurone disease.     

Early differentiating neuron in larval abalone (Haliotis kamtschatkana) reveals the relationship between ontogenetic torsion and crossing of the pleurovisceral nerve cords

Crossing of the pleurovisceral nerve cords in gastropods has supported the view that gastropods evolved by 180 degrees rotation between the ventral and dorsal body regions. Indeed, a rotation of this type occurs as a dramatic morphogenetic movement ("ontogenetic torsion") during the development of basal gastropods. According to a long-standing hypothesis, ontogenetic torsion in basal gastropods preserves an ancient developmental aberration that generated the contorted gastropod body plan. It follows from this reasoning that crossing of the pleurovisceral nerve cords during gastropod development should be mechanically coupled to ontogenetic torsion. The predicted mechanical coupling can now be examined because of the discovery of an early differentiating neuron in Haliotis kamtschatkana (Vetigastropoda) that expresses 5-hydroxytryptamine-like immunoreactivity. The neuron appeared to delineate the trajectory of the pleurovisceral nerve cords beginning before ontogenetic torsion. Before torsion, the neuronal soma is embedded in mantle epithelium at the ventral midline and two neurites extend anteriorly toward the apical sensory organ. Contrary to expectation, the two neurites of this cell did not cross-over during ontogenetic torsion because the soma of this mantle neuron shifted in the same direction as the rotating head and foot. Full crossing of the pleurovisceral nerve cords occurred gradually during later development as the mantle cavity deepened and expanded leftward. These results are consistent with a generalization emerging from comparative studies indicating a conserved developmental stage for gastropods in which the mantle cavity is localized to one side, despite a fully "post-torsional" orientation for other body components. Developmental morphology before this stage is much more variable among different gastropod clades.     

Adulthood and phylogenetic analysis in gastropods: character recognition and coding in shells of Lavigeria (Cerithioidea, Thiaridae) from Lake Tanganyika

In the study of gastropod shell morphology, determination of comparable ontogenetic stages is crucial, because all the states that various shell features go through during ontogeny are preserved on the shell. The protoconch/teleoconch transition and marks of episodic growth are among the few ways of defining discrete, comparable, growth stages. In gastropods with determinate growth the attainment of adulthood may provide additional shell markers permitting comparison among individuals and taxa. Adulthood is reflected in shell morphology in ways as diverse as shell deposition covering all the previous whorls and radically changing the shape of the shell through to slight changes in the trajectory of the suture. While the very prominent adulthood‐related changes of shell morphology have been used as systematic characters, the more moderate changes have not been studied in detail and their potential systematic value has been ignored. In this paper we give a detailed account of adult modifications of the shell appearing with cessation of growth. Our study group comprises eight closely related species of Lavigeria from Lake Tanganyika. We show that the ways adulthood is manifested are quite diverse. We describe eight characters of the aperture, the suture and the sculptural ornamentation. Character occurrence varies greatly among species. We show that characters appear in suites and that in many cases their appearance is connected to size. We use size as a proxy for adulthood and test whether character occurrence alone or its connection to size can help resolve species relationships. In both cases our characters confirm the monophyly of our ingroup and yield cladograms with various degrees of resolution of ingroup relationships. The coding method that yields the greater character congruence is the one that takes into consideration the connection between appearance of a character and size. This study demonstrates that ontogenetically correlated character transformations may nevertheless be phylogenetically independent. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 140 , 223?240.     

Evolution to the extreme: origins of the highly modified apical system in pourtalesiid echinoids

The apical system of the genus Pourtalesia displays a plate architecture that falls so far outside that typical of other echinoids that plate homologies remain problematic. A new approach using the Extraxial–Axial Theory (EAT) that develops homologies for the Echinodermata is proposed. The exploration of apical plate patterns throughout ontogenetic sequences shows that the typical holasteroid pattern found in the youngest specimens undergoes a series of disturbances that result in a multiple disjunction accompanied by isolation or disappearance of certain genital plates. We propose a new interpretation of the apical architecture of the genus that agrees with: (1) the plate addition processes as predicted by the EAT; (2) patterns observed in other genera of the Pourtalesiidae as well as in its sister‐group (plexechinids); and (3) the patterns known from Palaeocene holasteroids. In the context of the EAT, the genus Pourtalesia appears to represent the extreme in a reduction of the extraxial part of the body wall. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 140 , 137–155.     

Dynamic and static cord-structure: A new dielectrophoretic phenomenon

During the dielectrophoresis of cells or other particles it is observed that unusual distributions of the cells appear on the floor of the test chamber. In certain narrow frequency ranges, the distributions are cord-like, with the “cords” running more or less parallel to the electrode faces, but also developing highly branched structures. It is observed that the cells within the cords form “pearl-chains” along the field lines and across the cords. Within the cords, the cells move actively in spiralized paths. This the dynamic cord-structure is considered to be due to the combined action of negative dielectrophoresis, Brownian motion, and gravity. Outside the conditions requisite for dynamic cord formation, the cells are seen to assume conformations reflective of simple negative dielectrophoresis which merely causes them to amass into regions of low field strength. The theory for the effects is presented, together with experimental evidence for the kinetic behavior of the dynamic cord effect.     

Phylogenetic interpretation of ontogenetic change: sorting out the actual and artefactual in an empirical case study of centrarchid fishes

Hypothesized relationships between ontogenetic and phylogenetic change in morphological characters were empirically tested in centrarchid fishes by comparing observed patterns of character development with patterns of character evolution as inferred from a representative phylogenetic hypothesis. This phylogeny was based on 56–61 morphological characters that were polarized by outgroup comparison. Through these comparisons, evolutionary changes in character ontogeny were categorized in one of eight classes (terminal addition, terminal deletion, terminal substitution, non-terminal addition, non-terminal deletion, non-terminal substitution, ontogenetic reversal and substitution). The relative frequencies of each of these classes provided an empirical basis from which assumptions underlying hypothesized relationships between ontogeny and phylogeny were tested. In order to test hypothesized relationships between ontogeny and phylogeny that involve assumptions about the relative frequencies of terminal change (e.g. the use of ontogeny as a homology criterion), two additional phylogenies were generated in which terminal addition and terminal deletion were maximized and minimized for all characters. Character state change interpreted from these phylogenies thus represents the maxima and minima of the frequency range of terminal addition and terminal deletion for the 8.7 × 10³⁶ trees possible for centrarchids. It was found for these data that terminal change accounts for c. 75% of the character state change. This suggests either that early ontogeny is conserved in evolution or that interpretation and classification of evolutionary changes in ontogeny is biased in part by the way that characters are recognized, delimited and coded. It was found that ontogenetic interpretation is influenced by two levels of homology decision: an initial decision involving delimitation of the character (the ontogenetic sequence), and the subsequent recognition of homologous components of developmental sequences. Recognition of phylogenetic homology among individual components of developmental sequences is necessary for interpretation of evolutionary changes in ontogeny as either terminal or non-terminal. If development is the primary criterion applied in recognizing individual homologies among parts of ontogenetic sequences, the only possible interpretation of phylogenetic differences is that of terminal change. If homologies of the components cannot be ascertained, recognition of the homology of the developmental sequence as a whole will result in the interpretation of evolutionary differences as substitutions. Particularly when the objective of a study is to discover how ontogeny has evolved, criteria in addition to ontogeny must be used to recognize homology. Interpretation is also dependent upon delimitation within an ontogenetic sequence. This is in part a function of the way that an investigator ‘sees’ and codes characters. Binary and multistate characters influence interpretation differently and predictably. The use of ontogeny for determining phylogenetic polarity as previously proposed rests on the assumptions that ancestral ontogenies are conserved and that character evolution occurs predominantly through terminal addition. It was found for these data that terminal addition may comprise a maximum of 51.9% of the total character state change. It is concluded that the ontogenetic criterion is not a reliable indicator of phylogenetic polarity. Process and pattern data are collected simultaneously by those engaged in comparative morphological studies of development. The set of alternative explanatory processes is limited in the process of observing development. These form necessary starting points for the research of developmental biologists. Separating ‘empirical’ results from interpretational influences requires awareness of potential biases in the course of character selection, coding and interpretation. Consideration of the interpretational problems involved in identifying and classifying phylogenetic changes in ontogeny leads to a re-evaluation of the purpose, usefulness and information conveyed by the current classification system. It is recommended that alternative classification schemes be pursued.     

Geometric shell growth and mode of life of a Devonian chonetoidean brachiopod

Morphological and statistical analysis of the chonetoid species Kentronetes variabilis from the Lower Devonian (Lochkovian) of the Argentine Precordillera demonstrate ontogenetic changes and allometric relationships between characters. A special study was made of spine distribution, morphology, and growth, compared to valve growth. The first, inner, developed spines (pairs 1–1'and 2–2') continued to grow after development of the following outer pairs. The spacing of spines, their diameter, and the density of growth rings vary from beak to posterolateral margins following a specific 2ⁿ geometric growth factor, compared to the regular, almost linear growth of the valves, attested by growth lines. The linear growth rate of outer spines (pairs 3–3'and 4–4') can be 6–8 times more rapid than that of the shell on the valve margin. Ontogenetic changes in spine morphology are interpreted as a response to changes in the mode of life.     

Dine or Dash?: Ontogenetic Shift in the Response of Yellow Perch to Conspecific Alarm Cues

During their first year of growth yellow perch, Perca flavescens, undergo an ontogenetic niche shift from invertebrate feeding to piscivory. They also undergo a similar shift in their response to heterospecific alarm cues, switching from anti-predator to foraging behaviour. We conducted laboratory trials to determine whether yellow perch experience a comparable ontogenetic shift in their response to conspecific alarm cues. When exposed to either young-of-year (YOY) or adult perch skin extract, YOY perch responded with decreased time in motion and number of feeding attempts as well as increased time spent with spines erect and latency to first feeding attempt, all of which are indicative of an anti-predator response. Adult perch, when exposed to the same cues, responded with increased time spent moving and number of feeding attempts as well as decreased time spent with spines erect and latency to first feeding attempt, indicative of a foraging response. These data suggest that yellow perch undergo an ontogenetic niche shift in response to conspecific alarm cues.     

人胎儿脊髓神经干细胞的分离培养

本文旨在探讨是否能够从低温保存的流产儿分离培养出脊髓神经干细胞。将14周流产儿在4℃下保存,2、6和12h后取脊髓,将颈段、胸段、腰骶段分别进行无血清培养,并用胎牛血清诱导分化。用克隆培养的方法验证培养细胞的干细胞特性;用免疫荧光细胞化学的方法检测神经干细胞标志nestin及干细胞诱导分化后神经元标志MAP2、星形胶质细胞标志GFAP、胆碱能标志ChAT,并比较不同时间点以及不同部位分离的神经T细胞的差异。在各个时间点,从颈段、胸段、腰骶段脊髓均分离培养出具有连续增殖能力的神经球,其中腰骶段分离出的神经球数量最多,12h组各段分离出的神经球较2、6h组显著减少。各段培养中的神经球均为nestin阳性,诱导分化后均能够产生GFAP阳性星形胶质细胞、MAP2阳性神经元以及ChAT阳性胆碱能神经元。各段培养中的神经干细胞的克隆形成能力相似。以上结果表明,从低温保存的人胎儿能够分离培养出脊髓神经干细胞,这为基础研究以及未来治疗应用提供了新的细胞来源。     

Ovary cord structure and oogenesis in Hirudo medicinalis and Haemopis sanguisuga (Clitellata, Annelida): remarks on different ovaries organization in Hirudinea

In Hirudo medicinalis and Haemopis sanguisuga, two convoluted ovary cords are found within each ovary. Each ovary cord is a polarized structure composed of germ cells (oogonia, developing oocytes, nurse cells) and somatic cells (apical cell, follicular cells). One end of the ovary cord is club-shaped and comprises one huge apical cell, numerous oogonia, and small cysts (clusters) of interconnected germ cells. The main part of the cord contains fully developed cysts composed of numerous nurse cells connected via intercellular bridges with the cytophore, which in turn is connected by a cytoplasmic bridge with the growing oocyte. The opposite end of the cord degenerates. Cord integrity is ensured by flattened follicular cells enveloping the cord; moreover, inside the cord, some follicular cells (internal follicular cells) are distributed among germ cells. As oogenesis progresses, the growing oocytes gradually protrude into the ovary lumen; as a result, fully developed oocytes arrested in meiotic metaphase I float freely in the ovary lumen. This paper describes the successive stages of oogenesis of H. medicinalis in detail. Ovary organization in Hirudinea was classified within four different types: non-polarized ovary cords were found in glossiphoniids, egg follicles were described in piscicolids, ovarian bodies were found characteristic for erpobdellids, and polarized ovary cords in hirudiniforms. Ovaries with polarized structures equipped with apical cell (i.e. polarized ovary cords and ovarian bodies) (as found in arhynchobdellids) are considered as primary for Hirudinea while non-polarized ovary cords and the occurrence of egg follicles (rhynchobdellids) represent derived condition.     

Reconstructing the evolution of umbonal sculptures in the Unionida

‘Umbonal sculptures’ of freshwater mussels (Unionida), which ornament the early ontogenetic shell, have long been used for species identification. Specificity of these sculptures to higher taxonomic levels and their value for phylogenetic reconstruction are still under considerable scientific debate. In particular, the distribution of beak sculpture morphotypes across the unionoid phylogeny and, consequently, evolution of this character remain poorly understood. Based on an examination of 187 taxa, covering five of the six extant unionoid families, this study presents a new model of character evolution of umbonal sculptures in the order. Ten morphotypes were recognized and conceptualized into the cladistic characters sculpture presence and category. Optimization of sculpture presence on two recent hypotheses of palaeoheterodont phylogenetic relationships using the program Mesquite indicates a sculptured common ancestor of the extant Unionida, with multiple losses of the umbonal ornament occurring subsequently within the clade. Reconstruction of changes in sculpture category is ambiguous and demonstrates the need for further research into the evolutionary relationships of freshwater mussels in general and of their early ontogenetic sculptures in particular. Ambiguity is reduced in analyses applying a model with unequal costs of transformation between character states, which was derived from observations on intermediate forms and polymorphisms. These analyses suggest ‘V‐shaped’ or ‘nodulous’ sculpture as the plesiomorphic sculptural category for Unionida. The relatively low levels of homoplasy inferred for V‐shaped, pseudo‐radial and double‐looped sculptures suggest that these types may comprise useful guides to relationships within Unionida. The high degree of homoplasy of W‐shaped, pseudo‐concentric, wrinkled and single‐looped sculptures, on the other hand, renders these sculpture types less fit for such purposes.     

New scoring matrix for amino acid residue exchanges based on residue characteristic physical parameters

Based on residue characteristic physical parameters, a new scoring matrix, called EMPAR, for amino acid exchanges in proteins was obtained. When comparing protein sequences for detecting homologies, the use of this matrix in place of the Dayhoff log-odds matrix yields results that reflect the topological similarities in the proteins. The use of EMPAR is equivalent to the parametric correlates coefficient approach of Ooi and his colleagues. This matrix correlates at 0.63 with the Dayhoff matrix.     

Topological maps of protein sequences

A new method based on neural networks to cluster proteins into families is described. The network is trained with the Kohonen unsupervised learning algorithm, using matrix pattern representations of the protein sequences as inputs. The components (x, y) of these 20×20 matrix patterns are the normalized frequencies of all pairs xy of amino acids in each sequence. We investigate the influence of different learning parameters in the final topological maps obtained with a learning set of ten proteins belonging to three established families. In all cases, except in those where the synaptic vectors remains nearly unchanged during learning, the ten proteins are correctly classified into the expected families. The classification by the trained network of mutated or incomplete sequences of the learned proteins is also analysed. The neural network gives a correct classification for a sequence mutated in 21.5%±7% of its amino acids and for fragments representing 7.5%±3% of the original sequence. Similar results were obtained with a learning set of 32 proteins belonging to 15 families. These results show that a neural network can be trained following the Kohonen algorithm to obtain topological maps of protein sequences, where related proteins are finally associated to the same winner neuron or to neighboring ones, and that the trained network can be applied to rapidly classify new sequences. This approach opens new possibilities to find rapid and efficient algorithms to organize and search for homologies in the whole protein database.     

Post-pathological keel-loss compensation in ammonoid growth

Among the various pathologies documented in ammonoids, impairs affecting the apertural margin may have long-lasting sequelae on subsequent shell geometry. An interesting healing pattern, known as sculptural compensation, led to the permanent replacement of an ornament by adjacent sculptural elements. Moreover, in several ventrally impaired individuals the symmetry was preserved. Those developed annular ribs in place of any previous ventral ornamentation (keel, sulcus or smooth area). This phenomenon is known from diverse ammonite families. Monestieria resouchei (Monestier 1931), type species of 'Monestieriinae' Sapunov 1965, displays exactly that type of annularly-ribbed morphology and has been shown to be otherwise similar to species of Grammoceratinae Buckman 1904 occurring in the same beds, thus corroborating its pathological nature and leading to the rejection of that taxon. Now, keel absence in Praehaploceras Monestier 1931 and Buckmanites Guex 1973 cannot be explained by the same process as they do not have annular ribs. Moreover, the absence of any clue of malformation, their relative frequency and specific characteristics exclude the previously suggested synonymies with Pseudolioceras Buckman 1889 as equivalent pathological forms. In consequence, their rehabilitation is herein proposed. They should be included within Harpoceratinae Neumayr 1875.     

Chemico-structure of the organophosphatic shells of siphonotretide brachiopods

The organophosphatic shell of siphonotretide brachiopods is stratiform with orthodoxly secreted primary and secondary layers. The dominant apatitic constituents of the secondary layer are prismatic laths and rods arranged in monolayers (occasionally in cross-bladed successions), normally recrystallized as platy laminae. Sporadically distributed, interlaminar, lenticular chambers, containing apatitic meshes of laths and aggregates of plates and spherulites, probably represent degraded, localized exudations of glycosaminoglycans (GAGs) with dispersed apatite.
The shells of Helmersenia and Gorchakovia are perforated by canals with external depressions (antechambers) that possibly contained chitinous tubercles in vivo . The immature shell of Siphonotreta and most other siphonotretids is similarly perforated and pitted; but the mature part bears recumbent, rheomorphic, hollow spines that grew forward out of pits. Internally, spines pierce the shell as independent structures to terminate as pillars in GAGs chambers. Spines and pillars were probably secreted by collectives of specialized cells (acanthoblasts) within the mantle.
The shell of the oldest siphonotretide, Schizambon , is imperforate but the ventral valve has a pedicle foramen that lies forward of the posterior margin of the juvenile valve. This relationship characterizes all siphonotretides, suggesting that the pedicle, in vivo , originated within the ventral outer epithelium and not from the posterior body wall as in lingulides.     

Functional morphology of chonetidine (Brachiopoda) spines: biomechanical tests of a potential key innovation

Chonetidine brachiopods were a significant component of Paleozoic marine life; chonetidines experienced a major adaptive radiation during the Late Silurian and Early Devonian. The Chonetidina clade is united by the presence of spines on the hingeline but the function of these spines has not been clearly demonstrated. The present study performs a biomechanical experiment, using specimens of Neochonetes granulifera with and without artificial spines in a recirculating flume, to test if the spines may have inhibited entrainment in higher energy settings. Specimens with spines were less likely to be overturned or transported than those without spines, and were overturned or transported at significantly higher velocities than specimens without spines. Performance improved with the addition of another pair of spines. In addition, spinose specimens reoriented such that their commissures faced upstream. The results suggest that chonetidines could have survived in higher-energy environments, even if the brachiopods were not physically attached to the substrate. As such, it is functionally plausible that the evolution of hinge-spines may have possibly facilitated the adaptive radiation of the clade by enabling chonetidines to inhabit previously unoccupied habitats.     

Evolution of adaptation through allometric shifts in a marine snail

Variation in ontogenetic development among individuals may be a major contributor to morphological variation within species. Evolution of different growth trajectories might, for example, evolve as a response to varying ecological contexts of individuals living in different environments, or by life-stage or gender differences. The intertidal periwinkle Littorina saxatilis is strongly polymorphic in shell shape. We compared ontogenetic trajectories between life stages, local populations, and sexes to understand how different morphological end points are reached during ontogeny and what might cause these differences. Applying landmark-based geometric morphometrics, we captured shell shape variation for four Swedish populations of this species. We also derived a method to visualize ontogenetic trajectories described by the relationship of size to the multivariate shape space. We found that growth trajectories differed between individuals living in different habitats, as well as between sexes and maturity stages. Males living on rocky cliffs grew isometrically throughout life, whereas females from the same habitat switched from isometric growth as juveniles to allometric growth as adults. In contrast, males and females living on boulders grew allometrically as juveniles but changed to isometric growth at sexual maturity. Thus, in this species, ontogenetic growth seems influenced by habitat-associated selection as well as by gender and age-specific selection. These differing selection regimes result in ontogenetic shifts in allometry in three of the four groups examined.     

Ovary cords organization in Hirudo troctina Johnson, 1816 and Limnatis nilotica (Savigny, 1822) (Clitellata,Hirudinea)

In both examined species of Hirudinea there are paired spheroid ovisacs, and within each ovisac two convoluted ovary cords occur. The morphology of the cords is characteristic: their apical end is club-shaped, the central part is narrow and may contain developing oocytes, whereas the basal end of the cord is irregularly shaped and composed of degenerating cells. The ovary cords are built of somatic and germ-line cells; the latter are united into syncytial cysts. Each germ cell in such a cyst has only one stable cytoplasmic bridge connecting it to the central anuclear cytoplasmic mass, the cytophore. Initially all germ-line cells in a given cyst are morphologically identical, then the fates of cells diversify. Most of them become nurse cells and eventually degenerate; the rest continue meiosis, gather macromolecules, cell organelles and nutritive material and become oocytes. The oogenesis found in the species studied should be regarded as meroistic. Previtellogenic oocytes protrude from the cord into the ovisac lumen, whereas the vitellogenic ones float freely in the ovisac lumen. The somatic cells found in the ovary cords are: follicular cells which form the envelope of the cord and are also found among germ cells inside the cord, and one, huge apical cell that always is located at the top of the club-shaped end of the ovary cord. The apical cell has several characteristic features, e.g., it forms long cytoplasmic projections filled with intermediate filaments and it is connected to the neighbouring cells (both somatic and germ-line) via hemidesmosomes. We suggest that the apical cell forms the niche for maintaining germ and somatic stem cells. Generally, the organization of the ovary cords found in both studied species is broadly similar to those described in other hirudiniform leeches studied to date.