The structure of the areas pellucida and vasculosa of the early chick embryo (stages 11-29) was examined by light, transmission and scanning electron microscopy. The most striking feature of the endodermal cells of these areas is the presence of large intracellular yolk drops which are characteristic of the regions in which they are found; lipid-like homogeneous drops in the area pellucida, heterogeneously composed pleomorphic drops in the mid-region of the area vasculosa and granular drops at the periphery of the area vasculosa in the region of the sinus terminalis. On morphological criteria it is postulated that granular drops may arise by direct engulfment of extracellular yolk, but this does not appear to be true for pleomorphic or homogeneous drops. Since the apical junctions between endodermal cells across the yolk sac are tight, they seal off the extraembryonic compartment from the vitelline circulation and presumably prevent intercellular passage of the yolk constituents. Thus the endodermal epithelium must mediate the transport of nutrients from the yolk mass to the developing embryo. Endodermal cells exhibit a variation across the yolk sac in the presence and number of structures associated with uptake of extracellular materials. The mid-region of the area vasculosa appears to be the most endocytotically active region with an abundance of microvilli, bristle-coated pits and vesicles and apical canaliculi and vacuoles. There is a close association between the endoderm and vitelline blood vessels and this association is maintained, as the yolk sac develops, by the formation of small vessels juxtaposed between the vascular surface of the endoderm and the walls of the large vitelline vessels.

The endoderm of the chick yolk sac mediates the transfer of materials from the yolk mass to the embryonic circulation. There is little evidence of endocytotic activity in the area pellucida, but the endodermal cells of the area vasculosa possess many microvilli and bristle-coated pits and vesicles, as well as a canalicular system and vacuoles in the apical cytoplasm. Three tracers, horseradish peroxidase, ferritin, and latex spheres, were injected beneath the endoderm of both cultured embryos and embryos in ovo to study the pathway of uptake of extracellular materials. All tracers were sequestered in bristle-coated pits (200-500 nm in diameter) of the endodermal cells of the area vasculosa, but not those in the area pellucida. Both horseradish peroxidase and latex spheres (and probably ferritin) were incorporated into pleomorphic intracellular yolk drops through bristle-coated pits and vesicles, and then into apical vacuoles, which fuse with the intracellular yolk drops. Horseradish peroxidase and ferritin were also found within apical canaliculi. The apical junctions between endodermal cells prevented the intercellular passage of the tracers. A "topping-up" hypothesis is opposed whereby endodermal cells of the area vasculosa continually sequester extracellular yolk material, which is incorporated into a digesting intracellular yolk drop while, at the same time, digested yolk products are being transported across the vascular pole of the endoderm to the extraembryonic circulation and thence to the embryo.

The head kidney and spleen are major sites of haemopoiesis in fish; a secondary center is found in loose connective tissue of the intestine. In this study we determined the nature of gut-associated haemopoietic tissue in the goldfish, Carassius auratus, using light and electron microscopy. This tissue is a loose stroma of reticular cells and fibers vascularized by capillaries, venules, and arterioles. The cellular population includes lymphoblasts, small and medium-sized lymphocytes, plasmocytes, macrophages, and various granulocytes. The most abundant granulocyte is the mast cell, whose large granules stain with Alcian blue and toluidine blue. Heterophils are found in the intestinal connective tissue as well as two other granulocytes: one with ovoid granules having dense parallel lamellae and another with granules containing crystalline inclusions. Immature forms of both granulocytes were also noted. Macrophages containing phagocytosed debris were often located close to the epithelium; they were observed forming clusters with lymphocytes. The epithelium contained a number of migrating leucocytes including lymphocytes and lymphoblasts, macrophages, and heterophils. Although many granulocytes were found in the connective tissue, granulopoiesis does not seem to be a major function. Gut-associated haemopoietic tissue in goldfish resembles diffuse lymphoid tissue and may be involved in intestinal immune responses.

The ovarian cycle of the brook or 5-spined stickleback was studied histologically. Specimens were trapped between July, 1965 andAugust, 1966 in a shallow pond near London, Ontario.Spawning took place from mid-April to mid-July, when the water averaged 10OC. In early August, when the water averaged 20-22°C for prolonged periods, spawning ceased and the ovaries contained only oijgonia, atretic mature oocytes and empty follicles. Oijgonia and immature oiicytes were found throughout the year but only in the spring did the mature stages begin to appear.Oocytes for each year's spawning appear to arise mitotically from a residual stock of lightly basophilic oiigonia. As the oocytes mature, their basophilia increases and they acquire a vitelline membrane and follicular layer. The deeply basophilic cytoplasm of the early oijcyte becomes frothy, as oily droplets of primary yolk are laid down. Finally, i t becomes flled with eosinophilic secondary yolk in the mature Two types of atresia of developing eggs are common, especially during and after the spawning period. The first involves active phagocytosis of yolky material and hypertrophy of the follicular layer, leading to the formation of what could be called a "corpus luteum." In the second type of atresia, neither phagocytosis nor hypertrophy was seen and nothing resembling a corpus luteum was formed.egg.

The cyclic changes in the testis of the five-spined stickleback Eucalia inconstans (Kirtland) were studied histologically. Specimens were trapped between July 1965 and July 1967 in a shallow pond near London, Ontario.A three-dimensional microscopic study showed a main vas deferens and a system of primary, secondary and tertiary tubules.The testis cycle was divided into seven arbitrary stages. Spawning takes placc from mid-April to mid-July. This is followed by the division of primary spermatogonia which are located along the walls of the tubules, producing cysts of spermatogonia enclosed i n connective tissue which is surrounded by a thin epithelium. Both primary and secondary spermatocytes develop within these cysts. Breakdown of the cysts occurs with the development of spermatids and spermiogenesis occurs while spermatids are free in the tubules. Over-wintering of mature sperm takes place. Development of mature sperm from primary spermatogonia takes about 156 days.Germinal epithelium is absent but primary germ cells are believed to be those cells occupying the spaces between the tubules o f the testis. No tissue which might be implicated in hormone production was observed.Phagocytic invasion of the testis has been studied. Massive infiltration by phagocytes is believed to be responsible for the sudden increase in testis weight observed during spawning. These cells ingest sperm nuclei and groups of them have been observed in the lumen of the tubules and the vas deferens, probably on their way out of the body.The present paper is a histological study of the testicular cycle of Eucalia inconstuns (Kirtland) (Osteichythes: Gasterosteidae), the five-spined stickleback. It includes a stereoscopic description of the arrangement of the ducts and tubules; a study of cyst formation and structure in spermatogenesis; and an investigation by phase and ordinary light microscopy of the origin, activity, function, and fate of the phagocytes which invade the connective tissue of the testis.Earlier histological investigations of the teleostean testis have been mainly restricted to cyclic changes. Among the earlier workers was Turner ~('19) who outlined the seasonal cyclic changes in Perca flnvescens. Weisel ('43) attempted to trace correlations which might exist between the histology of the testis and the peculiar life history of the salmon Oncorhynchus nerlza which dies after the first spawning. Matthews ('38) spermatogenesis in Umbra limi, the mudminnow, with special reference to the behaviour of the spennatogonial chromosomes and the first maturation division. James ('56) described the histology of gonad changes in the blue gill Lepomis macrochirus, and the largemouth bass Huro salmoides. One of the more recent studies in teleosts was by Henderson ('62) who studied the annual cycle in the testis of the eastern brook trout Salvelinus fontinalis and made some preliminary observations in connection with the possible localization of endocrine tissue in the testis.In 1967, Braekevelt and McMillan described the ovarian...