COMPARATIVE STUDIES OF THE SPLEEN IN SUBMAMMALIAN VERTEBRATES : Ⅱ. MINUTE STRUCTURE OF THE SPLEEN, WITH SPECIAL REFERENCE TO THE PERIATERIAL LYMPHOID SHEATH

In an attempt to give a bird's-eye view concerning the minute structure of the spleen in lower vertebretes, the spleens of thirty species representing all the classes of the submammalian vertebrates, from Cyclostomata up to Aves, were examined, with special emphasis on the role of spleen in hemocytopoiesis. The chief findings are as follows : 1. An initial step in the evolution of spleen is seen in the spiral fold of the intestine of the larval lamprey, which shows the essential elements characteristic of spleen - the central artery, the perisrterial lymphoid tissue, and the red pulp. However, the splenic tissue in the spiral fold of the intestine in Cyclostomata is very primitive in structure and arrangement and lacks the sheathed capillary ( ellipsoid ). Moreover, it disappesrs almost completery in the adult stage. 2. At the evolutional level of Elasmobranchii, the spleen has the three major elements - the periarterial lymphoid hseath, the ellipsoid, and the red pulp. Moreover, the periarterial lymphoid sheath and the ellipsoid are so well developed that the histological picture of the spleen closely resembles that of mammalian spleen. 3. At the next higher evolutional levels represented by Teleostei, Urodela and Anura, the periarterial lymphoid tissue in the spleen is greatly reduced in amount and shows less tendency to from definite nodules