72 



GERM-CELL CYCLE IN ANIMALS 



the fourth day the amebocytes become separated 

 into wandering cells or their derivatives and repro- 

 ductive cells or tokocytes as indicated in the table. 

 The primordial archeocytes do not always occur 

 in the Clathrinidse as in Clathrina hlanca. In some 



Fig. 27. — A. Clathrina hlanca. Blastula stage showing posterior gran- 

 ular cells (p.g.c). (From Minchin, 1900.) B. Oogonium of a 

 sponge containing inclusions in the cytoplasm. {From Jorgensen, 

 1909.) C. Two oogonia in the ectoderm of Hydra fusca, each with 

 a cytoplasmic inclusion. {From Downing, 1909.) 



species there is only one; in others four or more 

 appear ; and sometimes they are entirely absent. 

 This last condition results from the formation of 

 amebocytes before the fixation of the larva. In 

 many other sponges the archeocytes migrate in at 

 the posterior pole and partially or entirely fill up 

 the segmentation cavity. Comparatively little is 

 known about the embrvologv of Hexactinellida and 



