42 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 Clathrinide as in Clathrina blanca. In some 
i 
a 
PEC 
Fic. 27.— A. Clathrina blanca. Blastula stage showing posterior gran- 
ular cells (p.g.c.). (From Minchin, 1900.) B. Odgonium of a 
sponge containing inclusions in the cytoplasm. (From Jérgensen, 
1909.) C. Two oégonia 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 embryology of Hexactinellida and 
