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Splenic white pulp of the Aleutian skate (Bathyraja aleutica), an elasmobranch, was investigated using light and transmission electron microscopy. The major cellular constituent was plasma cells, of the typical Marshalko type, characterized by well developed rough-endoplasmic reticulum and a Golgi complex. The morphology of the rough-endoplasmic reticulum was variable, being lamellar in some cells and spherical in others. Plasma cells with distended cisternae of rough-endoplasmic reticulum and cells with Russel bodies were often observed. Only a small number of lymphocytes were encountered. These findings indicate that the splenic white pulp is the major site for immunoglobulin production in this fish, thus confirming our previous immunocytochemical observation. Globules presumably containing immunoglobulin were found consistently associated with the Golgi complex. The secretion mechanism of immunoglobulin by plasma cells is discussed in connection with the globules.

Organogenesis in embryos (1 and 5 mm-disk widths) of the Aleutian skate,Bathyraja aleutica was described histologicaily in complete serial paraffin sections. Special attention was paid to localization of primordial germ cells (PGC) and development of lymphoid tissues. The embryo of 1 mm-disk width was at the somite stage with no evidence of organogenesis. PGC, gathered in the genital ridge, were seen in the somatic mesodermal layer as well as in the mesenchyme between the endoderm and splanchnic mesoderm of the 1 mm-embryo. Most of the visceral organs were developing in the embryo of 5 mm-disk width. With respect to the development of immune system, two pairs of thymus anlagen, filled with numerous thymic lymphocytes, were recognized in the pharyngeal region. A small focus of numerous immature blood cells appeared to be an anlage of the spleen was found between the stomach and the liver.

The structure and function of epithelial cells of the ductus vitellointestinalis in the Mustelus griseus embryo were studied through light and electron microscopic observations. The ductus vitellointestinalis is contained within the yolk stalk, which connects an embryo with the yolk sac, The internal surface of the ductus vitellointestinalis is a simple epithelium lining, and consists mainly of ciliated squamous cells and columnar cells with microvilli. Structural characteristics of the cells suggest that the ciliated squamous cellsmay aid in the transportation of yolk and that the columnar cells with microvilli may function as absorptive cells. It may be considered, therefore, that the ductus vitellointestinalis in the M. griseus embryo is engaged not only in the transportation of yolk, but also in the absorption of yolk being conveyed into the intestine.