1. Quail cells and chick cells can be distinguished by light and electron microscopy. In this paper, the derivation of mesenchymal tissues from the neural crest is discussed, based mainly on the quail-chick chimera technique of using quail cells as biological cell markers.
2. Neural crest cells down to the level of the 4th somite have the capacity to become mesectoderm in the normal development of birds. The mesectodermal cells differentiate into the bones and cartilages of the visceral skeleton, dermis of the face and ventral part of the neck, connective tissue of the salivary gland and pharyngeal derivatives such as the thyroid, parathyroid, and thymus. Stromal and endothelial cells of the cornea, odontoblasts, and smooth muscle cells of the branchial arch arteries are also mesectodermal derivatives.
3. Though the fate of neural crest cells in normal development is precisely dependent on their original site in the neural axis, environmental cues are also very important in determining their fate.

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Thymic epithelial reticular cells together with lymphocytes constitute the thymic parenchyma. Morphologically they are characterized by the stellate aspect forming a cytoreticulum, a large and vesicular nucleus, a tonofilament-rich cytoplasm, and abundance of desmosomes. Besides the role of providing a structural support for lymphocytes, epithelial reticular cells are responsible for the secretion of thymic hormones which promote lymphocyte proliferation and maturation. Clinical evidence is now appearing suggesting such thymic hormones' use as therapeutic agents against immunodeficiency diseases and cancer.

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Encapsulated sensory corpuscles of the Krause type were found in the mucosa of surgically removed human vocal cords.
The corpuscles were ellipsoidal structures of about 30 by 50μm and were located beneath the free edge in the mid-region along the intermembranous part. They contained a number of varicose nerve endings and lamellar cells. The lamellar cells had thin cytoplasmic lamellae which contained numerous cytoplasmic filaments and were interposed between the nerve endings. Attachment devices were frequently noted between the cytoplasmic lamellae and between the lamellae and nerve endings. Half-desmosomes were also noted along the plasma membrane of the lamellar cells. The intercellular space was filled with amorphous electron lucent material and contained a few collagen fibrils. Ladder-like filamentous structures were frequently encountered in the intercellular space.
The location of the corpuscles at the free edge of the vocal cords suggests that the endings may receive the bilateral touch of the vibrating part of the cords in order to give sensory information for the control of the movement of the cords in phonation.

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The notochord was examined by light, transmission and scanning electron microscopy, in 25 pouch young opossums (Didelphis virginiana) ranging in age from 6hr to 25 days post natum. In the newborn opossum, the notochord forms a thin cylinder within developing vertebral centra and presents diamond-shaped expansions in regions of the developing intervertebral discs. The opossum notochord differs from that of other species in that it appears to consist of a heterogeneous population of light and dark cells, and also in the thinness of the notochordal sheath which never attains the degree of development seen in other animals. The sheath is made up of thin cytoplasmic processes which arise both from the light and dark cells. The processes cross a mucopolysaccharide-filled space and then expand to form a thin cytoplasmic sheath that forms the external boundary of the space. The matrix of the cartilage immediately adjacent to the cytoplasmic sheath shows an increased concentration of microfilaments as compared to the cartilage further from the notochord.
During postnatal development the notochordal tissue in the centra of the vertebral bodies decreases, while in the regions of the developing discs, there is a corresponding increase. At the end of the first week, notochordal tissue in the vertebral centra is represented only by a thin cylinder of mucopolysaccharide which contains cell fragments. In subsequent weeks, as ossification of the centra occurs, both the cartilage and notochordal tissue are replaced by bone. Notochordal tissue in the intervertebral regions contributes to the formation of the nucleus pulposus of the intervertebral discs. Despite its short gestation period of only 12 1/2 days, the basic structure and fate of the notochord in the opossum is similar to that of other mammals.

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The hindgut of Aëdes togoi larvae adapted to various external salinities was studied by light and electron microscopy. The larvae's pyloric chamber and the ileum of the hindgut are surrounded by a thick muscular layer and lined with a very thin unspecialized epithelium. There is no detectable difference in the ultrastructural organization of the epithelium between Seawater-adapted and freshwater-adapted larvae. The rectum and the anal canal are greatly dilated and densely tracheated and have a very thick, highly specialized epithelium. The rectal epithelium is characterized by shorter apical and longer basal infoldings closely associated with mitochondria, and a particulate coating on the apical infoldings. The intercellular spaces and the basal channels are tightly closed in the rectal epithelium of freshwater-adapted larvae, whereas widely dilated in that of 100 and 200% seawater-adapted larvae. The rectum seems to reabsorb water from excretory fluid when larvae are in hyperosmotic media. The anal canal epithelium possesses well-developed apical infoldings, which are coated with small particles and intimately associated with mitochondria. The basal infoldings are short. No perceptible difference in the ultrastructural organization of the epithelium was observed between seawater-adapted and freshwater-adapted larvae. The lumina of the apical infoldings of the anal canal epithelium in seawater-adapted larvae, however, demonstrated a strongly positive reaction to the cytochemical assay for chloride ions, whereas the reaction was very weak in freshwater-adapted larvae. Unlike salt-water-mosquito larvae of the other species, in which the rectum is considered to be concerned with the formation of hyperosmotic urine, the anal canal in A. togoi larvae seems to play an important role in excretion of excess ions in the haemolymph when adapted to hyperosmotic media.

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An electron microscope study was made to elucidate the manner in which reticular fibers form in the developing spleen of the chick embryo. On the 7th day of incubation, the splenic reticulum consisted of dendritic reticular cells without reticular fibers. From the 9th day onward, the intercellular gaps between processes of adjacent reticular cells were expanded, becoming occupied by fine flocculent materials. With the growth of the embryos, these materials were aggregated into felt-like bands, exhibiting the characteristics of mature reticular fibers. In the interstices of the reticulum, there appeared microfibrils of 8-30nm in diameter. The reticular cells possessed numerous free ribosomes and polysomes, well-developed Golgi apparatuses, dilated rough-surfaced endoplasmic reticula, and many coated vesicles. The coated vesicles contained fine flocculent materials as presumable fiber precursors. The vesicles were enlarged as they approached the cell periphery, where they appeared to discharge their contents by reverse pinocytosis. Some vesicles close to the cell membrane contained ruthenium red-positive materials which were considered to be glycosaminoglycan in nature and to compose the matrix of fibers. These materials also seemed to be secreted by reverse pinocytosis, being densely accumulated in the intercellular gaps. The differentiation of fibrous proteins into felt-like reticular fibers or striated microfibrils seems, therefore, to be intimately associated with the amount of glycosaminoglycan.

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Following subcutaneous injections of 6-OHDA into neonatal rats, the central noradrenaline (NA) system was examined by fluorescence and electron microscopy from 14 to 42 days after birth. Repeated administrations of 6-OHDA induced a serious confusion in the distribution of the central amines. NA fluorescence was reduced in the neurons of the main part of the locus ceruleus, while a considerably strong fluorescence remained in the neurons of the ventral part of the nucleus and the subceruleus. Coarse varicosities markedly increased in number in the pons and cerebellum, particularly in the motor nucleus of the trigeminal nerve, where they surrounded individual motor neurons. On the contrary, a depletion of NA was obvious in the entire cerebral cortex, even though a few fluorescence varicosities were occasionally seen in the entorhinalis and piriformis.
Exemplifying ultrastructural alterations in the pons after 6-OHDA administrations, small cored vesicles and large ones were demonstrated in the terminals adjacent to the trigeminal motor neurons by means of potassium permanganate fixation, but they were relatively few. Besides, it was characteristic of the synaptic cleft of the axo-somatic synapses to be so extended as to double its usual width. Occasionally, deposits of a dense material were seen in the loosened gap and glial processes invaded among the terminals contacting to the motor neurons. Thus, the increased NA around the motor neurons was probably accumulated as a soluble substance in the extravesicular compartments.
The cerebral cortex showing a chronic reduction of NA fluorescence after 6-OHDA administrations revealed an occurrence of neuronal necrosis. Necrotic neurons accompanied with the membranous lemellar bodies (MLB) were mainly scattered in the deeper layers of the cingulate cortex and the entorhinalis, though some were seen in the superficial layers of the temporal cortex. It seemed that the membrane structure of the MLB located in the terminals and the glial processes initially arose from the subsurface cisterns in the adjacent necrotic neuron. These alterations in the cerebral cortex were somewhat similar to those observed in the nigro-neostriatal dopamine system, suggesting again that amine input may be important to maintain the normal function of the cortical neurons.

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After a monocular injection of tritiated proline into the vitreous chamber of the eye, the distribution of labeled retinal termination in the hypothalamus of the crabeating monkey was investigated by autoradiography. The results indicated that a few retinofugal fibers might terminate bilaterally in the lateral hypothalamic areas through the level of the anterior commissure to the infundibulum as well as the suprachiasmatic nuclei. The labeled fibers to the lateral hypothalamic area seemed to leave the dorsocaudal border of the optic chiasma in a dorsolateral direction and then coursed both rostrally and caudally above the supraoptic nucleus: these fibers might progressively diminish in number at more rostral and caudal levels. Further, small clusters of developed silver grains were found bilaterally in the caudal part of the lateral preoptic nuclei.

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