Somato-dendritic synapses in the paraventricular organ of two anuran species

Summary The ultrastructure of the paraventricular organ ofXenopus laevis andRana esculenta has been investigated. Special attention has been paid to contact regions between nerve cells of the paraventricular organ and other nerve cells.According to the results of the glutaraldehyde-osmium tetroxide fixation as well as the zinc iodide-osmium method, it appears that the perikarya of several nerve cells of the paraventricular organ make synaptic contacts with dendrites of other nerve cells. It is suggested that the presence of the somato-dendritic synapses is a morphological indication for the sensory function of the nerve cells.Thanks are due to Prof. Dr. P. G. W. J. van Oordt for his interest and the many helpful discussions, and to Mr. H. van Kooten and his co-workers for their skilful photographic assistance.     

Hypothalamic neurosecretion and metamorphosis in Xenopus laevis

Summary Normal and propylthiouracil (PTU) treated Xenopus laevis tadpoles were fixed during all stages of metamorphosis and sagittal sections were stained with aldehyde fuchsin (AF) or pseudoisocyanine (PIC). Whereas AF positive neurosecretory material could only be demonstrated in the preoptic nucleus from late prometamorphosis, increasing amounts of PIC positive material were found in cells of the dorsal part of the preoptic region from early premetamorphosis. The development of these cells correlated with that of the thyrotropic cells and the thyroids. Likewise, signs of hyperactivity in thyroids and thyrotropic cells of PTU treated larvae were accompanied by a depletion of the dorsal PIC positive cells. In the ventral preoptic region PIC positive cells developed from late prometamorphosis in control larvae, but failed to do so in PTU treated animals. It is argued that the differentiation of the PIC positive cells is largely dependent on thyroid hormones; that the dorsal PIC positive cells may produce a thyrotropin releasing factor; and that the function of these dorsal cells is inhibited by thyroid hormones.The authors thank Dr. P. G. W. J. van Oordt for his active interest and helpful advices, Miss Tineke Aafjes for technical assistance and Mr. H. van Kooten for making the photographs.     

Hypothalamic control of the pars intermedia in Xenopus laevis tadpoles

Summary In Xenopus laevis tadpoles the relation between a paired nucleus of bio-amine producing neurons in the caudal hypothalamus and the pars intermedia of the hypophysis was studied.Treatment of the animals (stage 49 to 50 of Nieuwkoop and Faber's normal table) with reserpine caused aggregation of the skin melanophores within one hour, followed by redispersion five to six hours after the beginning of the experiment. This was at exactly the same time as the bio-amines in the caudal hypothalamus disappeared. However, the drug was ineffective if the nuclei had been removed. This indicates that reserpine acts via these nuclei and does not influence the skin melanophores directly.It was concluded that the initial aggregation of the melanophores may be the result of a reduced extrusion of MSH from the pars intermedia, caused by an increased output of a MIF by the bio-amine producing nuclei. The redispersion was explained by assuming that the bio-amines were depleted and the nuclei stopped with the extrusion of the MIF. This does not mean that the production of a MIF is the only function of the paired bio-amine producing nucleus in the caudal hypothalamus.The author thanks Prof. Dr. P. G. W. J. van Oordt for his helpful comments and criticism. Mr. J. H. I. J. M. ten Berge and Mr. E. W. A. Kamperdijk provided great assistance during the course of the experiments. Mr. H. van Kooten made the diagram and the photograph.     

Some aspects of the fine structure of the statocysts of the molluscsPecten andPterotrachea

Summary The statocyst ofPecten is composed of hair cells and supporting cells. The hair cells bear kinocilia and microvilli at their distal ends and the supporting cells bear microvilli. The cilia have a 9+2 internal filament content, and arise from basal bodies that have roots, basal feet and microtubular connections. Two different ciliary arrangements are described, one with a small number of cilia arranged in a ring, and another with many more cilia arranged in rows. Below the hair cells are probable synapses. A ciliated duct connects to the lumen of the static sac and passes through the centre of the static nerve. The hair cells in the statocyst ofPterotrachea bear kinocilia and microvilli. The possible importance of cilia and microvilli in the transduction process is discussed.We would like to thank ProfessorJ. Z. Young for bringing specimens ofPterotrachea from Naples and also the staff of the Stazione Zoologica for the provision of specimens, Dr.M. Land for providing specimens ofPecten, the Science Research Council (U.K.) for providing the electron microscope used in much of the study and also for a grant to one of us (V.C.B.), and Mrs.J. Parkers and Mr.R. Moss and Mrs.J. Hamilton for much photographic and technical assistance.     

Hypothalamic neurosecretion and metamorphosis in Xenopus laevis

Summary Tadpoles of Xenopus laevis were treated with propylthiouracil from the second half of prometamorphosis. Sagittal sections of the head region were stained a.o. with pseudoisocyanine. The goitrogen caused a degranulation of neurosecretory cells in the dorsal part of the preoptic region of the hypothalamus, suppressed the development of ventral neurosecretory cells and of the outer zone of the median eminence, stimulated the thyrotropic cells in the adenohypophysis, caused a hypertrophy of the thyroids, and impaired metamorphosis. Returning the animals to tap water had reciprocal effects and restored the normal activity of the hypothalamus, adenohypophysis and thyroid glands. It is concluded that thyroid hormones exert a morphogenetic influence upon hypothalamic centres and the outer zone of the median eminence and that a negative feed back relation exists between the thyroids on the one hand and the dorsal neurosecretory cells and the thyrotropic cells on the other.The author thanks Prof. Dr. P. G. W. J. van Oordt for his active interest and helpful advice, and Miss Tineke Aafjes for technical assistance.     

Hypothalamic neurosecretion and metamorphosis inXenopus laevis

Summary The frontal brain was removed from premetamorphic larvae ofXenopus laevis. Part of the animals was kept in a 0.01% propylthiouracil (PTU) solution for 21, 30 or 90 days, together with unoperated controls. Others were reared in tap water for the same length of time. The hypothalamus, the pituitary and the thyroids were studied in sagittal sections stained with aldehyde fuchsin or pseudoisocyanine (PIC).Operated tadpoles lacking the telencephalon only, could not be distinguished from unoperated controls. Extirpation of the preoptic region of the hypothalamus lowered the activity of the thyrotropic (TSH) cells and of the thyroids, and blocked metamorphosis. In animals with intact PIC-positive cells in the dorsal preoptic region, PTU induced a degranulation of these cells, stimulated the TSH cells and caused a strong hypertrophy of the thyroids. It is concluded that these neurosecretory cells in the dorsal preoptic region produce a thyrotropin-releasing factor (TRF), indispensable for the endocrine regulation of metamorphosis, and that the thyroid hormones have a negative feed-back influence upon the TRF cells.In part of the animals lacking the rostral hypothalamus, new PIC-positive cells were observed to develop immediately behind the preoptic region. In these animals PTU was somewhat less effective than in the animals with an intact dorsal preoptic region. This means either that the newly differentiated PIC-positive cells have a TRF function or that the thyroids can exert a direct negative feed-back upon the TSH cells. The latter possibility is supported by the fact that even in the absence of any neurosecretory cell PTU stimulated the thyroids.Blocking of thyroid hormone production by PTU inhibited the differentiation of the hypothalamus, of the median eminence and of the portal vessels. It may be deducted that during metamorphosis the general morphogenetic effect of the thyroid hormones stimulates the differentiation of the structures necessary for the augmentation of the TSH activity.The author thanks Prof. Dr.P.G.W.J. van Oordt for his active interest and helpful advices, MissTineke Aafjes for technical assistance and Mr.H. van Kooten for making the diagrams and photographs.     

The fine structure of the olfactory mucosa and nerve in the teleost Carassius carassius L.

Summary Morphological studies on teleost olfactory mucosa confirm the findings of previous authors regarding the general arrangement of conventional cell types, viz. receptor, sustentacular, mucous and basal, but teleosts show certain distinct differences. The receptor cells have the general mammalian bipolar shape but their peripheral dendrite does not project beyond the epithelial surface. In addition to numerous typical cilia, an exceptional ciliary formation was observed in which the filaments, instead of forming individual cilia, are grouped together in clusters and are enveloped in a single limiting membrane.At the junction between the finger-like process and the mucosal fold myelinated nerve fibres are observed within the subepithelial stroma.Within the postero-medial zone of the mucosa is a conspicuous well-differentiated new cell type. A thick rim of electron-dense cytoplasm, bounded by an outer trilaminar membrane, encloses prominent foliate (leaf-like) organelles, a basal nucleus, numerous mitochondria and vacuolar spaces. These foliaceous cells communicate with the external environment through a small stoma, their close association with epithelial components suggesting a possible secretory or absorptive function. Their intricate morphology, however, suggests that they may be receptors, but their role and neural connections still require definition.Supported by Grant 5 RO 5 TWOO 154-02 from the National Institutes of Health, United States Public Health Service.The authors are indebted to Dr. A. S. Wilson for his helpful criticism and gratefully acknowledge the photographic technical assistance of Mr. J. Simmons and Mr. S. Frank.     

A specialized trophospongium in large neurons of Leptodora (Crustacea)

Summary A specialized type of trophospongium has been described in large nerve cells of the cerebral ganglion of the planktonic crustacean Leptodora kindtii. It consists of three parts (Fig. 6). The first is, as a rule, long; it is composed of an infolding of the plasma membrane of the neuron, an intercellular space and a slender process of a glial cell. The second segment from the end of the glial process to the beginning of the X-body is always short; it is characterized by the presence of a desmosome-like junction. The third part consists of a labyrinth of cisternal spaces lined by membranes which are continuous with the infoldings of the surface membrane of the nerve cell.This investigation was supported by U.S.P.H.S. Grant NB 02145-04. The skillful assistance of Mrs. Cynthia Jones, Mr. Stanley Brown and Mr. Douglas Gasner in different phases of this work is gratefully acknowledged.     

Nerve supply and distribution of cholinesterase activity in the external nose of the mole

Summary Nerve supply and the distribution of cholinesterase activity were studied in the skin of the external nose of seven moles using a simplified Bielschowsky-Gross silver method and Koelle's histochemical technique.The sensory units of the mole's nose or the organs of Eimer are surrounded by blood sinuses which facilitate their movements during mechanical stimulation. All nerve fibres of the plexus deep to the basal cell layer of Eimer's organ ultimately become intra-epidermal endings. Contrary to the findings of earlier investigators, Merkel's discs, Pacinian corpuscles and Ruffini corpuscles have not been observed at the base of Eimer's organ. In the superficial layer of the plexus, the Schwann sheath cells increase in number, undergo modification and give a positive cholinesterase reaction.It is suggested that the organ of Eimer, the specialised nerve plexus deep to it and the surrounding blood sinus together constitute the touch receptor on a similar principle of transmission by leverage as in the tactile hair or the intermediate ridge of the papillary ridge.The role of the intra-epidermal nerve endings of the mole's nose as tactile receptors is disputed. A suggestion is made that tnese nerves may constitute pain and temperature receptors and that several modalities of sensation may be carried to the brain along one and the same medullated axon.We gratefully acknowledge the technical assistance of Miss Jill Hocknell. Our thanks are also due to Mr. C. J. Duncan and the staff of the Photography Department for their aid with the photographic work. We are particularly grateful to Mr. D. Burgess of the Ministry of Agriculture and Fisheries for kindly supplying us with live moles. One of the authors (N.C.) acknowledges an equipment grant from the Royal Society.     

The fine structure of the eye of the mollusc Pecten maximus

Summary The retina of Pecten maximus is divided into two light sensitive layers forming the distal and proximal retinae. The cells from these layers have different electrophysiological responses, the distal cells giving primary off responses, and the proximal cells giving on responses. The receptor surfaces of the distal retinal cells are formed from lamellae produced by the outer membranes of flattened cilia. These cilia have a basal body, basal foot, no root system and a 9 + 0 internal filament content. Each cell gives rise to an axon from its distal side, and this process goes up to the basement membrane, which is present below the cellular lens, passes along beneath it, and joins the distal optic nerve. The receptor part of the proximal retinal cells is formed from a vast array of microvilli. Each of these cells also bears one or two cilia with a probable 9 + 0 internal filament complement and no roots. The proximal cells give rise to axons, forming the proximal optic nerve. Below the proximal retina is a reflecting layer, the argentea, and below this is a pigment cell layer.We would like to acknowledge the advice and encouragement of Professor A. F. Huxley, Professor J. Z. Young and Dr. E. G. Gray. — We would like to thank Mrs. J. I. Astafiev for drawing Fig. 1, Mr. S. Waterman for photographic help and Miss C. Martin for clerical assistance.     

The fine structure of sensory receptor processes in the auricular epithelium of the planarian,Dugesia tigrina

Summary The marginal epithelium of the lateral auricles of the planarian, Dugesia tigrina, includes a cell type with surface cilia and microvilli, a basal nucleus, and dense cytoplasm containing secretory vacuoles, Golgi elements, mitochondria and ribosomes. Through channels within the epithelial cytoplasm, cellular processes, interpreted as extensions of neurosensory receptor cells located in the subepidermis, project to the surface. The receptor processes, containing microtubules, mitochondria, vesicles and an agranular tubular reticulum, project beyond the epithelial cell surface; one or two cilia each emerge from a basal body in the apex of the projection. Close to the point of emergence to the epithelial surface, each cylindrical receptor process is surrounded by a collar-like septate junction between adjacent plasma membranes. The cilia of the projections differ from those of the epithelial cells in diameter, density of matrix and in the banding patterns of the rootlets. A few projections appear with the apex and basal body retracted below the epithelial surface. The possible function of these ciliated processes in sensory reception is discussed.This work was supported by Grant No. SO 1 FR 5369 from the U.S. Public Health Service to the University of Illinois at the Medical Center.I thank Dr. J. P. Marbarger, Director of the Research Resources Laboratory, for use of the electron microscope facilities, Miss Irena Kairys for technical help, Miss Marie Jaeger for assistance with photography, and Mr. Robert Parshall for the drawing.To Professor Arthur Wagg Pollister, I respectfully dedicate this article on the occasion of his retirement from Columbia University.     

Scanning electron microscopic studies of cilia

Summary A simple method for the preparation of ciliated epithelia for study with the scanning electron microscope is described. Ciliary groups are well preserved and it is possible to discern individual cilia and work out their numbers and orientation. Following scanning electron microscopical study some of the material was prepared for transmission electron microscopy and the ultrastructure of the tissue was found to be surprisingly well preserved. The tracheal epithelium of the rabbit, the olfactory epithelia of the goldfish and the rabbit, and the sensory epithelia in the statocyst of a cephalopod mollusc were examined with the scanning electron microscope to demonstrate the possibilities of the method. Acknowledgements. We would like to thank Professor J. Z. Young for his continued interest and support. The scanning electron microscope was purchased with a grant provided by the Science Research Council to Dr. Boyde, Mr. R. Willis helped in the initial stages of the study, Mr. G. Savage provided help with the goldfish material, Mr. S. Waterman provided much photographic assistance, and Mrs. N. Finney the secretarial assistance.     

Distinct axonal projections from two types of olfactory receptor neurons in the middle chamber epithelium of <Emphasis Type="BoldItalic">Xenopus laevis</Emphasis>

Most vertebrates have two olfactory organs, the olfactory epithelium (OE) and the vomeronasal organ. African clawed frog, Xenopus laevis, which spends their entire life in water, have three types of olfactory sensory epithelia: the OE, the middle chamber epithelium (MCE) and the vomeronasal epithelium (VNE). The axons from these epithelia project to the dorsal part of the main olfactory bulb (d-MOB), the ventral part of the MOB (v-MOB) and the accessory olfactory bulb, respectively. In the MCE, which is thought to function in water, two types of receptor neurons (RNs) are intermingled and express one of two types of G-proteins, Golf and Go, respectively. However, axonal projections from these RNs to the v-MOB are not fully understood. In this study, we examined the expression of G-proteins by immunohistochemistry to reveal the projection pattern of olfactory RNs of Xenopus laevis, especially those in the MCE. The somata of Golf- and Go-positive RNs were separately situated in the upper and lower layers of the MCE. The former were equipped with cilia and the latter with microvilli on their apical surface. These RNs are suggested to project to the rostromedial and the caudolateral regions of the v-MOB, respectively. Such segregation patterns observed in the MCE and v-MOB are also present in the OE and olfactory bulbs of most bony fish. Thus, Xenopus laevis is a very interesting model to understand the evolution of vertebrate olfactory systems because they have a primitive, fish-type olfactory system in addition to the mammalian-type olfactory system.     

The fine structure of the light organ of the glowworm Lampyris noctiluca

Summary The four main parts of the glowworm light organ are the cuticle, the hypodermis, the photocyte layer and the reflector cell layer. The hypodermis is one cell thick and it contains hypodermic glands. These glandular cells have a lumen that opens to the outside of the cuticle. Projecting into the lumen are numerous microvilli. Between the hypodermis and photocytes are typical insect tunicated nerve fibres. They pass down between the photocyte and reflector layer cells. They do not appear to innervate the photocytes and they are thought to innervate adjacent muscle fibres or to be sensory. Tracheoles are commonly present between the photocytes but no tracheolar end organs are found. The photocytes contain amorphous granules, mitochondria, photocyte granules and a vesiculated reticulum. All, except the mitochondria, are absent from the reflector layer and so probably have some connection with light production. The reflector layer contains glycogen granules, clear spaces thought to be the sites of urate crystals, and membranous granules. The latter granules are sometimes found in photocytes adjacent to the reflector layer whilst amorphous granules are sometimes absent from these adjacent cells. So a cell layer with some features of the photocyte and reflector layer cells is present. These morphological findings are discussed with regard to the unknown function of the reflector layer and the control of light emission. Acknowledgments. We would like to thank Professor J. Z. Young and Dr. E. G. Gray for their advice and encouragement, Mrs. Jane, Astafiev for drawing fig. 1, Mr. S. Waterman for photographic assistance, Miss Cheryl Martin for secretarial assistance, and many colleagues for help in collecting specimens of glowworms.     

II. Ultrastructure of the type B sense organ of the specific lateral line system of Gymnarchus niloticus

Summary The type B cutaneous receptor represents one of the three kinds of specific organs of the lateral line system of Gymnarchus niloticus (Szabo and Barets, 1963). Electron microscopic observations reveal that the elementary unit (Fig. 18) of the type B organ consists of a well organized assembly of different epithelial elements grouped around each sensory cell. Several such units compose a type B organ innervated by a single myelinated nerve fiber.The cytoplasm of the sensory cell is characterised by a deep invagination lined by long and densely packed microvilli covered by a jelly-like substance. This jelly substance of allongated stylet form is situated in an intraepidermal cavity, overlaid by vacuolised epithelial cells oriented perpendicularly toward the external epidermal surface.Certain morphological characteristics of the organ B allows the conclusion that this organ is one of the possible electroreceptors of the Gymnarchus niloticus.This work was supported by grant No. 659535 of the Direction de Recherches et Moyens d'Essais (D.R.M.E.) to Dr. Szabo.This work was carried out on the electronmicroscope of the Service de Microscopie Electronique du Laboratoire de Médecine Experimentale (RCA) et du Laboratoire d'Histologie Normale et Pathologique du Système Nerveux (Siemens).With the technical assistance of Miss L. Seguin and Mr. C. Pennarun.     

An immunocytochemical method for the visualization of tubulin-containing structures in the egg of Xenopus laevis

Summary Tubulin can be isolated and purified from Xenopus laevis egges through modification of Olmstedt's (1970) tubulin isolation method, viz. by repeating the vinblastin precipitation step after resuspension of the sediment in a detergent-containing stabilizing medium. By this we overcome the deleterious influence of the yolk granules in the isolation procedure. From 1 l of Xenopus laevis eggs 25 mg VB-paracrystals can be obtained. The apparent molecular weight of the purified tubulin is 52,800. Antiserum against the purified Xenopus VB-paracrystals, raised in 2 Chinchilla rabbits, cross-reacts in immunodiffusion tests in agar gels with rat brain tubulin and with tubulin isolated from Xenopus laevis eggs by the described procedure. Specific indirect fluorescence staining and appropriate control reactions reveal that cilia of Tetrahymena pyriformis, cytoplasmic networks in cultured mouse Leydig cells, as well as mitotic spindles and nuclear regions in paraffin sections of Xenopus laevis blastulae, react with the antibodies against Xenopus laevis egg tubulin as well as with monoclonal antibodies against pig brain tubulin.These results provide additional evidence for the view that tubulin antibodies are neither species nor tissue specific and show that under appropriate conditions tubulin containing structures can be visualized in paraffin sections.     

Fine structure of cilia in rat cerebral cortex

Summary Cilia have been demonstrated on granular neurons and astroglial cells in the fascia dentata, a part of the hippocampal region, in the rat. Every granular cell seems to possess one cilium, which shows an 8+1 pattern in the greater part of its length. This 8+1 pattern is shown to result from the displacement of one peripheral doublet of a 9+0 cilium into the middle of the cilium. The neuronal cilia have a two-centriole basal organization, and fine rootlets radiate from the basal body proper into the cytoplasm. The possible function and significance of these cilia are discussed on the basis of earlier literature.This study was supported in part by Grant NB 02215 of The National Institute of Neurological Diseases and Blindness, U.S. Public Health Service, in part by The Norwegian Research Council for Science and the Humanities. This aid is gratefully acknowledged. I am greatly indebted to Dr. Th. Blackstad for encouragement and advice during this study, to Mrs. J. L. Vaaland, Mr. B. V. Johansen and Mr. E. Risnes for technical assistance, and to Dr. B. Afzelius for valuable discussions.     

Fluorescence studies on neural structures and endocrine cells in the pancreas of the cat

Summary With the use of the Falck-Hillarp histochemical technique for the detection of monoamines, nerve fibre fluorescence is observed throughout the tail of the pancreas of the cat and the arrangement and distribution of the nerve fibres can be studied in both the exocrine and endocrine tissue. In the exocrine pancreas, adrenergic nerve fibres innervate arterioles, larger veins and major pancreatic ducts. Adrenergic nerve fibres also appear to terminate on the non-adrenergic nerve cell bodies of the intrapancreatic ganglia. In the islets of Langerhans, adrenergic nerve fibres innervate both the endocrine cells and blood vessels. Some of the islet cells exhibit fluorescence with the Falck-Hillarp technique and these cells have been identified as alpha cells. In animals treated with reserpine, the fluorescence in nerve fibres and in alpha cells is absent.The author wishes to thank ProfessorG. C. Schofield and Dr.G. C. Smith for their encouragement and valuable criticism during the course of this study. The assistance of MissJ. Bennett and MissW. Kemp and the photographic help of Mr.J. S. Simmons, F.R.P.S., are gratefully acknowledged. The diagram was drawn by MissS. Flett.     

Evidence for corticotropin releasing factor (CRF) synthesis in the preoptic nucleus of Xenopus laevis tadpoles

Summary For the study of the hypothalamo-hypophysial system of Xenopus laevis tadpoles, hypothalamic lesions were made by means of the electrocoagulation technique. Lesioning of the ventral region of the preoptic nucleus resulted in a decrease of the number of ACTH cells in the pars distalis of the pituitary gland and in a diminution of the PAS-positive reaction of these cells. In addition, regeneration of the neurosecretory cells of the ventral region of the preoptic nucleus observed 6 weeks after lesioning was accompanied by the reappearance of normal PAS-positive ACTH cells in the pars distalis. It is suggested that the neurosecretory cells of the ventral region of the preoptic nucleus of Xenopus laevis tadpoles are related to the ACTH synthesizing cells, probably by producing CRF.Dedicated to Prof. Dr. med. W. Bargmann on the occasion of his 70th birthdayThe authors thank Prof. Dr. J.C. van de Kamer for his interest, Prof. Dr. P.G.W.J. van Oordt for his many helpful comments, and Messrs. H. van Kooten, E. van der Vlist, J.J. van der Vlis and M.C.A. van Pinxteren for preparing the illustrations     

Electron microscopic studies on the pars intermedia of the ferret

Summary The structure of the pars intermedia of the ferret has been studied with the electron microscope, with particular reference to the morphology of the glandular cells and their innervation. Two types of cell were found. The predominant cell is ovoid in shape and contains membrane-bound vesicles of varying size (1,000–5,000 Å) and density, the most electron-dense of which are associated with the Golgi region. The nucleus is indented and the cytoplasm contains rough endoplasmic reticulum. The second cell type is often associated with the colloid material and is elongated or stellate-shaped with long processes which extend between the predominant cells. It is devoid of cytoplasmic vesicles and has a poorly defined Golgi apparatus. Certain other structural features of this cell such as microvilli, cilia or cytoplasmic microfilaments are reminiscent of ependymal cells.Numerous nerve endings are observed throughout the pars intermedia, making synaptic contact with the predominant cell type. The majority contain vesicles with an electron-dense core measuring 750 Å; less frequently terminals contain dense granules measuring 1,000 A or more. Both also contain small electron-lucent vesicles (200–400 Å); occasionally terminals containing only the latter type are found. The pattern of innervation in the ferret is thus comparable to that previously observed in the cat, rather than that seen in rodents or monkeys, and the implications of this finding are discussed.We are indebted to Prof. Sir Solly Zuckerman, O. M., K. C. B., F. R. S., for his help and guidance and to Mr. J. Wallington for his unfailing technical assistance.