42 PETER OKKELBERG 



middle third of the coelomic cavity and not with its whole length 

 as in older larvae. 



Where the coelomic epitheUum covers the germ gland it is 

 thicker than elsewhere and its nuclei are much more numerous. 

 These nuclei are ovoid in shape and are usually so oriented that 

 their long axes are parallel to the long axis of the body and to 

 one another. Each contains, besides a chromatin network, a 

 large nucleolus, usually near its center (fig. 35). Many of the 

 larger chromatin granules lie against the inner surface of the 

 nuclear membrane. No membranes are visible between the 

 cells of the peritoneum, but cell limits are indicated by the 

 lessened thickness of the cytoplasm between the nuclei. All the 

 cells of the germinal peritoneum are similar and in no case was 

 the peritoneal membrane found to be interrupted so that the 

 germ cells might be said to form a part of it. Evidence has 

 already been presented to show that these two kinds of cells are 

 of different origin and that there is no genetic relationship between 

 them. That the follicle cells are derived from peritoneal cells, 

 as in earher stages, is indicated by the similarity in structure 

 of their nuclei and by the fact that in various places peritoneal 

 cells are migrating inward from the peritoneal covering of the 

 gland to take part in the formation of follicles around the cysts 

 and individual germ cells. When a germ cell or cyst lies against 

 the peritoneum, the cells of the latter are usually heaped up 

 around it in such a way as to suggest that they will eventually 

 enclose it altogether. Some of the peritoneal cells also migrate 

 into the interior of the gland without at first coming into rela- 

 tion with germ cells. This usually happens where the peritoneal 

 membrane is indented, in which case the cells separate from the 

 apex of the indented portion (fig. 35). Whether or not all 

 these cells finally attach themselves to germ cells and form 

 follicles for them is not known, although in some cases they 

 seem to become associated with the more deeply situated germ 

 cells. For a time at least, they make up most of the stroma 

 of the gland, although part of the latter enters the gland from 

 the mesenchyme dorsal to it. The nuclei of these few mesen- 

 chymal stroma cells may be distinguished from the peritoneal 



