THE NORMAL FINE STRUCTURE OF OPOSSUM TESTICULAR INTERSTITIAL CELLS

The interstitial tissue of the opossum testis includes interstitial or Leydig cells, macrophages, and small cells which morphologically resemble mesenchymal cells. The latter are thought to give rise to mature interstitial cells. The most prominent feature of the interstitial cell cytoplasm is an exceedingly abundant agranular endoplasmic reticulum. This reticulum is generally in the form of a meshwork of interconnected tubules about 300 to 450 A in diameter, but occasionally it assumes the form of flattened, fenestrated cisternae resembling those of pancreatic acinar cells, except for the lack of ribonucleoprotein particles on the surface of the membranes. The interstitial cells vary considerably in their cytoplasmic density. The majority are quite light, but some appear extremely dense, and in addition usually have a more irregular cell surface, with numerous small pseudopodia. These differences may well reflect variations in physiological state. Cytoplasmic structures previously interpreted as "crystalloids" consist of long bundles of minute parallel tubules, each about 180 A in diameter, which seem to be local differentiations of the endoplasmic reticulum. The mitochondria are rod-shaped, and contain a moderately complex internal membrane structure, and also occasional large inclusions that are spherical and homogeneous. The prominent juxtanuclear Golgi complex contains closely packed flattened sacs and small vesicles. The results of the present study, coupled with biochemical evidence from other laboratories, make it seem highly probable that the agranular endoplasmic reticulum is involved in the synthesis of the steroid hormones produced by the interstitial cell. This finding therefore constitutes one of the first functions of the agranular reticulum for which there is good morphological and biochemical evidence.     

Fine structure of the Malpighian tubules in the housefly, Musca domestica

The epithelium of the Malpighian tubules in the housefly is comprised of four distinct cellular types. Type I cells are characterized by the presence of intimate associations between infoldings of basal plasma membrane and mitochondria. On the luminal surface, cytoplasm is extended into microvilli which contain mitochondria. Membrane-bound vacuoles in the cytoplasm seem to progressively accumulate granular material. Type II cells have dilated canaliculi. Microvilli lack mitochondria. The Type III cell has not been reported previously in Malpighian tubules. It has very well-developed granular endoplasmic reticulum which contains intracisternal bundles of tubules. Cytoplasm contains numerous electron dense bodies. Type IV cells occur in the common duct region of the Malpighian tubules. Mitochondria do not extend into the microvilli.     

The fine structure of testicular interstitial cells in the adult golden hamster with special reference to seasonal changes

Summary The fine structure of the testicular interstitium was studied in normal adult golden hamsters sacrificed in the reproductive season (spring and summer) and in the winter. The Leydig cells in the reproductively active testes contain abundant endoplasmic reticulum (ER) and numerous mitochondria. The ER occurs in the form of flattened cisternae and tubules, the former prevailing. The cisternae are extremely extensive and are partly granular and partly agranular, their ends being continuous with the tubular reticulum. Mitochondria intervening between the cisternae are closely associated with the agranular portions of the latter. Adjacent to the Golgi complex and continuous with the centrosome a unique filamentous body with a dense laminar core is often observed. In the regressive testes, the Leydig cells show a great reduction of cytoplasmic volume and a remarkable decline of the organelles, especially agranular tubules. The possible functional significance of the tubular and cisternal ER with the associated mitochondria is discussed in relation to the biosynthesis of androgens. Macrophages appear to constitute another important population of the interstitial cell clusters.This study was supported in part by a grant from the National Science Council, the Republic of China     

Hormone-induced differentiation of agranular endoplasmic reticulum in the interstitial cells of the mouse testis

Summary The administration of chorionic gonadotrophin to prepuberal mice results in precocious maturation of the testicular interstitial cells. The cytoplasm of the nine-day-old cells is characterized by abundant lipid droplets, large numbers of glycogen particles and mitochondria. By contrast, the membranous organelles are poorly developed.Human chorionic gonadotrophin brings about mobilization of lipid droplets and glycogen particles, and differentiation of large areas of agranular endoplasmic reticulum.The present observations are in agreement with the reports that human chorionic gonadotrophin increases the secretion of testosterone and that the agranular endoplasmic reticulum is the site of storage of steroid and of the enzymes involved in the biosynthesis of androgens.     

THE FINE STRUCTURE OF ACANTHAMOEBA CASTELLANII : I. The Trophozoite

The fine structure of the trophozoite of Acanthamoeba castellanii (Neff strain) has been studied. Locomotor pseudopods, spikelike "acanthopodia," and microprojections from the cell surface are all formed by hyaline cytoplasm, which excludes formed elements of the cell and contains a fine fibrillar material. Golgi complex, smooth and rough forms of endoplasmic reticulum, digestive vacuoles, mitochondria, and the water-expulsion vesicle (contractile vacuole) are described. A canicular system opening into the water-expulsion vesicle contains tubules about 600 A in diameter that are lined with a filamentous material. The tubules are continuous with unlined vesicles or ampullae of larger diameter. Centrioles were not observed, but cytoplasmic microtubules radiate from a dense material similar to centriolar satellites and are frequently centered in the Golgi complex. Cytoplasmic reserve materials include both lipid and glycogen, each of which amounts to about 10% of the dry weight.     

On the ultrastructure of follicles and isolated follicular granulosa cells of porcine ovary

Summary The endoplasmic reticulum in granulosa cells of primary, secondary, and small tertiary follicles of the porcine ovary is sparse and largely of the granular type.In granulosa cells of large tertiary follicles the endoplasmic reticulum shows distinct signs of proliferation. Some cells even contain whorls of endoplasmic reticulum membranes, essentially of the agranular variety.Direct continuity between endoplasmic reticulum membranes of the granular and agranular type as well as the continuous increase in agranular membranes suggest that these membranes may originate from the granular membranes.Granulosa cells isolated from large tertiary follicles by microdissection and keptin vitro show essentially the same ultrastructure as granulosa cells of intact large tertiary follicles.Some lipid droplets appear to be localized in cavities of the endoplasmic reticulum. It is suggested that the droplets contain precursor material for steroid hormone synthesis.Finally, the development of the agranular endoplasmic reticulum including the appearance of whorls in some granulosa cells of large tertiary follicles indicates that steroid synthesis may occur in such follicular granulosa cells.Read at the Meeting of the Swedish Society for Pathology in Umeå, September 25, 1965 (Bjersing, 1966).This investigation was supported by grants from the Swedish Medical Research Council (Projects No. 13 X-78-01, 12 X-78-02, and 12 X-78-03).     

Fine structure of the corpuscles of stannius in the toadfish.

The micro-anatomy of the corpuscles of Stannius of the toadfish, Opsanus tau, an aglomerular marine teleost, has been studied by light and electron microscopy. The corpuscles are composed of extensively anastomosed cords of epithelial cells which maintain intimate contact with blood capillaries. Most of the epithelial cells contain acidophilic granules which also show a positive reaction with the periodic acid-Schiff technique and aldehyde fuchsin. On the basis of fine structural criteria, three cell types can be recognized. The granular cells contain abundant quantities of granular endoplasmic reticulum, ribosomes, Golgi apparatus with prosecretory granules, coated vesicles, polymorphic mitochondria with lamellar cristae, filaments, microtubules, a cilium, a variety of lysosome-like dense bodies, glycogen particles, lipid droplets, secretory granules and intranuclear lipid-like inclusions. One variety of agranular cell (type I) is characterized by the total absence of secretory granules, but it contains large amounts of granular endoplasmic reticulum and ribosomes, conspicuous profiles of Golgi apparatus, coated vesicles and sometimes an abundance of glycogen. Another variety of agranular cell (type II) has poorly developed cytoplasmic organelles. The perivascular space between the capillary and parenchyma contains connective tissue cells and abundant nerve fibers. The different types of epithelial cells observed in the corpuscles of Stannius of this fish may represent functional stages of the secretory cycle in a single cell type.     

The fine structure of the light organ of the New Zealand glow-worm Arachnocampa luminosa (Diptera: Mycetophilidae).

The swollen distal tips of the Malpighian tubules of the glow-worm Arachnocampa luminosa constitute the light organ. The ventral and lateral surfaces are covered by a tracheal ‘reflector’ and the nervous supply to the light organ comes from the ganglion in the penultimate segment. Fine nerve terminals, axons, and glial cells can be seen in close proximity to the basal surface of the cells of the light organ. The epithelial cells of the light organ are large, the cytoplasm dense, homogeneous and acidophilic. The cytoplasm gives a strong positive reaction for protein. The cytoplasm contains a high density of free ribosomes, patches of dense material, smooth endoplasmic reticulum, glycogen and scattered microtubules. Mitochondria are numerous; they are large, randomly distributed and packed with fine cristae. These cells lack the features characteristic of Malpighian tubule epithelial cells; infolding of the apical and basal cell surfaces is reduced and the cytoplasm contains few organelles. These cells do not contain secretory or photocyte granules and the grainy cell matrix is thought to be the luciferin substrate. Oxygen is supplied via the tracheal layer (which may have secondary reflecting properties) and light production controlled by neurosecretory excitation either directly via synapses, or by hormones. There are no other reports of Malpighian tubules of insects producing light and the fine structure of these cells is distinct. Thus, the swollen distal tips of the Malpighian tubules of the glow-worm undoubtedly constitute a unique luminescent organ.     

THE ENDOPLASMIC RETICULUM OF GASTRIC PARIETAL CELLS

An electron microscopic survey has been made of the gastric parietal or oxyntic cell of the human, cat, beaver, dog, hamster, rat, mouse, and bat, and of the corresponding cell type in two species of frog, two species of toad, and the horned lizard. A feature consistently found in the parietal cells of the mammals or their equivalent in the lower vertebrates is the agranular endoplasmic reticulum, which takes the form of branching and anastomosing small tubules approximately 200 to 500 A in diameter, sometimes expanded into flattened cisternae. In mammalian parietal cells this form of the endoplasmic reticulum is found only in limited amounts, but in the corresponding secretory cells of the amphibia and reptilia the tubular agranular reticulum is abundant. It is believed to comprise a more or less continuous system of channels, but owing to their tortuous course only short profiles are seen in thin sections. Immediately subjacent to the plasmalemma at the free surface, the cytoplasm is relatively free of organelles but is occasionally traversed by the agranular reticulum, which appears to be continuous at some points with the cell surface. The possible participation of the agranular endoplasmic reticulum in hydrochloric acid secretion is discussed.     

Ultrastructure of the pseudohermaphrodite rat testis

Summary The interstitial cells of the pseudohermaphrodite rat testis are both hypertrophic and hyperplastic. The cytoplasm is characterized by smooth endoplasmic reticulum which is abundant and variable in form. Mitochondria are numerous and large with tubular cristae and occasional inclusions. Structural features of the Leydig cells indicate potential for increased steroid synthesis. The presence of large numbers of mast cells in the intertubular area is confirmed.Small seminiferous tubules lack advanced germinal elements. Additional connective tissue and myoepithelial layers produce a thickening of the limiting membrane. Some myoepithelial cells are atypical with an electron translucent cytoplasm and nuclei with dense peripheral chromatin. No spermatogenic cells beyond the cap phase of the spermatid are observed. The cytoplasm of Sertoli cells contains large lipid droplets and degenerating germ cells.The authors are greatly indebted to Drs. A. J. Stanley, J. E. Allison, and L. G. Gumbreck for kindly providing the animals for this study.     

Fine structural changes in Sertoli and Leydig cells during the reproductive cycle of the ground squirrel, Citellus lateralis

During spermatogenesis in sexually mature ground squirrels Leydig and Sertoli cells were morphologically well differentiated. For Leydig cells the most prominent organelles were lipid droplets, mitochondria with tubulo-vesicular cristae and abundant agranular reticulum organized as a mass of anastomosing tubules. These morphological criteria suggest that the Leydig cells were steroidogenically active. Sertoli cells exhibited a topographical distribution of certain organelles with basal regions containing stacks of granular reticulum, and large areas of agranular reticulum. The cytoplasm surrounding maturing germ cells contained numerous microtubules, and an adluminal layer of spermatids at a certain stage of spermiogenesis became enveloped by Sertoli cytoplasm containing an enormous proliferation of agranular reticulum. The presence of these organelles in Sertoli cells suggests that during spermatogenesis they are active in the synthesis of proteins and steroids. In particular the mass of agranular reticulum surrounding late stage spermatids indicates that steroids may be required for spermatid maturation and/or spermiation. By contrast Leydig and Sertoli cells observed during testicular regression, when only spermatogonia remain in the seminiferous tubules, had undergone structural changes. Leydig cells were still numerous and large with abundant agranular reticulum that was now organized as a loose assemblage of single unbranched tubules. Sertoli cells were drastically reduced in both cytoplasmic volume and content of organelles.     

Electron microscopic observations on the involution of the human corpus luteum of menstruation

Summary The involution of the granulosa lutein cell in the human corpus luteum is characterized by a dilatation of agranular endoplasmic reticulum vesicles and tubules. This process continues until the whole cell is filled with large vacuoles and the cytoplasm is reduced to thin strands between the vacuoles. The contents of the latter are of low electron density in contrast to the very electron dense lipid droplets in vascularization, bloom and early involution phase. Light microscopical evidence shows that the contents of the vacuoles must be lipid and the lower electron density might be explained by the relative decrease in phospholipids and increase in cholesterol and cholesterol esters during involution. Simultaneously processes of focal cytoplasmic degradation resulting in autophagic vacuoles occur in the cells. These lead in some cases to the formation of residual bodies which can be identified with lipofuscin granules. Finally, the degenerating cells disintegrate and part of the debris, including the lipofuscin granules are phagocytosed by macrophages, the so-called fluorocytes of Hamperl. During involution an amorphous substance with some protofilaments and collagen fibrils is deposited in the spaces between the shrinking lutein cells. This is the fibrohyalin material which will form the bulk of the corpus albicans.We are grateful for the assistance given by Dr. J. M. Moyes of the Women's Hospital, Crown Street, Sydney and the staff of King George V. Hospital at Sydney with the supply of the material.     

OVARIAN STEROID CELLS : II. The Lutein Cell

The lutein cells of the rabbit exhibit fine structural variations during their life-span of 28 to 30 days. A systematic examination of the corpus luteum reveals that cellular distinctions may be recognized during the first, second, and third stages of pregnancy. The agranular endoplasmic reticulum reveals vesicular, tubular, and cisternal profiles after fixation with each of the following fixatives: glutaraldehyde, osmium tetroxide, and permanganate. The osmolality of the fixing solutions was varied with sucrose and recorded with an osmometer in order to determine the effect of osmotic concentration on the intracellular membranous profiles. It was determined that vesicles and short, branched tubules of similar structure are present in the agranular reticulum when the osmolalities are 300 to 800 milliosmols (iso-osmotic considered 300 milliosmols). At 900 milliosmols, the vesicular or tubular lumen is obliterated. Intracellular membrane profiles do not exhibit interconversions due to hyperosmotic fixative solutions. The agranular endoplasmic reticulum is randomly distributed as short tubular profiles during the first third of pregnancy. A continuity between these membranes and irregular, electron-opaque lipid masses is evident. When physiological and histochemical data indicate that the lutein cell may be storing sterol precursors, cytological observations show that the agranular endoplasmic reticulum exists in a more organized pattern within the cytoplasmic matrix. Vesicular and short tubular, circular aggregations as well as whorled cisternal patterns surround the larger, less electron-opaque lipid droplets. Surface views of cisternal agranular endoplasmic reticulum exhibit tubular extensions, accentuating the continuity between these two profiles. During the progress of pregnancy, the lutein cell increases in diameter, and accumulates both lipid inclusions and aggregations of intracellular membranes. The agranular endoplasmic reticulum may be peripherally packed and arranged parallel to the cell surface during later stages. In the postpartum, degenerating lutein cell, large myelin figures are present which form from the agranular endoplasmic reticulum. These cellular events are discussed in relation to lutein cell activity, including both secretion of product and storage of precursors.     

ULTRASTRUCTURAL STUDIES OF PINUS PINASTER NEEDLES: THE ENDODERMIS

The endodermis in the needles of Pinus pinaster was examined with light and electron microscopy. The endodermis is composed of very long, radially flattened cells, filled with a large central vacuole, which contains spherical dense bodies whose concentration decreases from the ends of the cell to the middle part. They are individually surrounded by a fine granular matrix. The central vacuole is bounded by a thick tonoplast. Other small, clear vacuoles are limited by a thin tonoplast. The parietal cytoplasm contains relatively few ribosomes, long slender chloroplasts, and lipid bodies. The smooth endoplasmic reticulum is highly developed along the tangential walls and frequently connected, or apposed, to the plasma membrane. Numerous primary pit fields are seen in the radial walls which are lignified and in the tangential walls; the latter exhibit a characteristic loosening of the outer layer of the wall. The lipid bodies are connected to endoplasmic reticulum tubules. The role of the endodermis in the active transport of water inside the needle is discussed in reference to previous physiological studies. The chemical composition of the vacuolar dense bodies is as yet unknown.     

COTTON EMBRYOGENESIS: THE EARLY DEVELOPMENT OF THE FREE NUCLEAR ENDOSPERM

An electron microscope study was made of the central cell and the development of the free nuclear endosperm surrounding the zygote and synergids during the first three days after pollination. The cytoplasm of the central cell, concentrated around the partially-fused polar nuclei, contains many ribosomes, mitochondria and large, dense, starch-containing plastids, some dictyosomes and lipid bodies, and long, single cisternae of rough endoplasmic reticulum (RER) that frequently terminate in whorls. Dense, core-containing microbodies are closely associated with the RER. After fertilization the cytoplasm of the 2-and 4-nucleate endosperm shows an increase in number of dictyosomes, and in amount of RER which becomes stacked in arrays of parallel cisternae. Cup-shaped plastids are associated with many long, helical polysomes. Perinuclear aggregates of dense, granular material also appear after fertilization. Granular aggregates and helical polysomes disappear after the first few divisions of the primary endosperm nucleus. During the second and third days of development there is an increase in dictyosome number and RER proliferation, and endosperm nuclei become deeply lobed. Concurrently, there is a sharp decline in the starch and lipid reserves of the central cell and elaborate transfer walls are formed at the micropylar end of the embryo sac and on the outer surface of the degenerating synergid. The transfer walls contain groups of small, membrane-bound vesicles, and are associated with large numbers of mitochondria and with the smooth endoplasmic reticulum.     

Structural and ultrastructural features of boar seminal vesicles

The morphological features of boar seminal vesicles were examined by light and transmission microscopy. Boar seminal vesicles consist of glandular tissue arranged in multiple lobules containing a system of ramified secretory tubules. The secretory tubules are composed of a mucosa formed by an epithelium and an underlying lamina propria and, are surrounded by a muscular layer. The epithelium is made up of columnar cells and occasional basal cells. Mast cells are frequently found among epithelial cells. Three types of columnar cells, considered different stages of the secretory cell cycle, are present: principal cells, clear cells and dense cells. Principal cells are functionally differentiated cells characterised by abundant mitochondria, great development of the rough endoplasmic reticulum and presence of secretory granules in their cytoplasm. The apical surface of many principal cells shows apical blebs filled with PAS-positive material. No acid mucosubstances are detected. Microvilli cover the apical surface except in the apical blebs. Dense cells, arranged between principal cells, are also functional differentiated cells but with signs of cellular degeneration. Clear cells are an initial differentiated stage of columnar cells and are characterised by the presence of a poorly developed rough endoplasmic reticulum and by the absence of secretory granules. Proliferating cells are present among columnar cells. Basal cells contain scarce cytoplasm, few organelles and no secretory granules. The lack of mitotic activity in these cells suggests that they do not act as precursors of columnar cells.     

The ommatidium of the termite mastotermes darwiniensis

An electron microscope study of the compound eye of the termite Mastotermes darwiniensis shows that the ommatidia have both primitive and specialized features. The ommatidia are of the apposition type with eight similar retinular cells, a fused rhabdom, irregularly orientated rhabdomere tubules and no tracheae. The retinula cells contain lipid. The cone cells have unusual processes, which thicken towards the basement membrane and contain rough endoplasmic reticulum, granules and dense bodies. These processes may act as a primitive transport system for nutrients.     

OBSERVATIONS ON THE FINE STRUCTURE OF LUTEIN CELLS : II. The Effects of Hypophysectomy and Mammotrophic Hormone in the Rat

Corpus luteum formation was induced in 26-day-old rats which were subsequently hypophysectomized and injected with mammotrophic hormone (MH, LTH). Sections of corpora lutea from these animals were examined with the electron microscope and compared with similarly prepared (Caulfield's fixed, Araldite embedded) corpora from normal pregnancy and from controls, the latter consisting of corpora prior to hypophysectomy and corpora from uninjected rats 7 to 14 days after hypophysectomy. Lutein cells from corpora lutea of injected animals and of normal pregnancy are characterized by abundant, tortuous, tubular agranular endoplasmic reticulum and by mitochondria, many of which are disc-shaped with dense matrices and both villiform and lamelliform cristae. The endoplasmic reticulum is most abundant in lutein cells from pregnant animals, in which cells it is in the form of thin, highly tortuous tubules. The form of the lipid droplets seen in cells of stimulated animals varies greatly. Marginal foldings of the lutein cells on the perivascular space were found in all instances. Lutein cells from hypophysectomized animals have a less highly developed agranular endoplasmic reticulum. The mitochondria have irregular outlines and a relatively lucid matrix. The lipid droplets in these cells show less tendency to be extracted, but are not so large or abundant as in the cells of onset controls. Granules believed to contain lipid pigments are common in the lutein cells of these control animals. It is suggested that lutein cells from corpora lutea which are actively secreting progesterone may be readily distinguished from lutein cells from non-active corpora by means of the multiple characteristics enumerated. It is further suggested that mammotrophic hormone has a general effect on the metabolism of lutein cells rather than solely affecting a specific organelle, the abundance or composition of which may be the limiting factor in the production of progesterone.     

Ultrastructural evidence for a secretory function in the “gland cells” of the marine red algaBotryocladia pseudodichotoma (Rhodymeniaceae)

Summary Clusters of small, pyriform gland or vesiculate cells protrude from the inner cortex into the inflated bladders ofBotryocladia pseudodichotoma. These multinucleate cells contains a large, central vacuole surrounded by dense cytoplasm. The lobed chloroplasts lack phycobilisomes and contain concentric rather than parallel thylakoids. Irregularly shaped, osmiophilic bodies of undetermined composition are abundant in the cytoplasm and are also released into the vacuole. The cell is characterized by abundant Golgi apparatus. The Golgi bodies appear to form by coalescence of endoplasmic reticulum derived vesicles and tubules. The Golgi apparatus produces large, electron transparent vesicles that contain an osmiophilic, fibrous material. This material is secreted into a diffuse wall layer which appears to be continually sloughed. Thus, the vesiculate cells ofBotryocladia possess an ultrastructure characteristic of granulocrine secretory cells, and it is suggested that they produce the slime contained within the bladders of this genus.     

ELECTRON MICROSCOPY OF THE VOLVOCACEAE AND ASTREPHOMENACEAE

Lang, Norma J. (U. Texas, Austin.) Electron microscopy of the Volvocaceae and Astrephomenaceae. Amer. Jour. Bot. 50(3): 280-300. Illus. 1963.—Clonal cultures of Gonium sociale, G. pectorale, Pandorina morum, Eudorina elegans, Eudorina sp., Volvulina steinii, V. pringsheimii, Platydorina caudata, Pleodorina illinoisensis, P. californica, Volvox aureus, V. tertius, V. globator, V. barberi, and Astrephomene gubernaculifera representing the Volvocaceae and Astrephomenaceae in the Volvocales were examined with the electron microscope and their ultrastructure compared. The ultrastructure of the various organelles is basically similar in the species studied and no increase in cellular complexity is found to accompany the evolutionary trends evidenced in the Volvocaceae. The ultrastructure of a colonial cell is basically that of Chlamydotnonas. A cytoplasmic membrane having a unit membrane structure encompasses a cell and is continuous with the double-membraned flagellar sheaths. The flagella contain the typical 9 + 2 fibril arrangement with the 2 axial fibrils terminating in a cylinder at the flagellar base and the 9 peripheral pairs continuing into the cytoplasm as a basal body. The organelles comprising the cytoplasm are: mitochondria with plate-like cristae; dictyosomes composed of stacks of agranular cisternae; small, rough or smooth-surfaced vesicles; an endoplasmic reticulum of granule-bearing and agranular tubules, lamellae and broad cisternae; vacuoles which are either contractile, contain fine granular and fibrillar material, or have dense contents probably representing polyphosphate; lipid bodies; and dense granules 100–150 A which have been called ribosomes. The finely granular nucleoplasm is surrounded by a porous, double-membraned nuclear envelope and contains a centric nucleolus composed of dense, spherical granules. The outer membrane of the nuclear envelope bears granules and may have granular extensions into the perinuclear cytoplasm. Each extension appears to encompass one or several dictyosomes and has been termed an “amplexus.” The amplexi are agranular on the surface contiguous to a dictyosome. A double-membraned chloroplast envelope is continuous around the single, cup-shaped chloroplast. The basic chloroplast units are discs closed at each end, occurring in stacks of varying number parallel to the envelope. The presumed proteinaceous matrix of the basal pyrenoid is penetrated by elongated, tubular elements which connect with the lamellar discs. Multiple rows of granules, associated with individual discs, form the anterior stigma within the chloroplast envelope. The colonial matrix is not a structureless, mucilaginous material uniting the cells in colonies, but it has rather a highly complex structure especially around the periphery of the colony and the flagellar channels. The apparent substitution of a fibrillar layer of the colonial matrix for the discrete compact cell wall, such as is found in Chlamydomonas, implies a greater degree of complexity in the evolution of these colonial genera than is generally assumed.