44 PETER OKKELBERG 



c. Larva 27.5 mm. long. Although the histological features 

 of the gland have not changed from those of the preceding stage, 

 this particular larva was selected for study because certain 

 cytological structures in the germ cells are more clearly shown. 

 One of these is the centrosphere, which differs from that of the 

 20-mm. larva, in which it was last referred to, in that it shows a 

 distinct centrosome. The vitelline body, as variable in position 

 as in the 20-mm. larva, is visible. It is now more compact and 

 stains black in sections treated with iron haematoxyhn. In fav- 

 orable sections the outline of each individual germ cell may be 

 faintly seen in the cysts (fig. 38). The germ cells are in various 

 phases of mitosis, but most of them are in the resting phase, as in 

 previous stages. Those of a cyst do not divide sunultaneously, 

 but several cells within the same cyst may be in different phases 

 of division, while others are in a resting condition (figs. 39 and 

 67). The cysts range from two to over thirty cells and there 

 are a great number of isolated cells. 



d. Larva 30 mm. long. In most of the larvae of this stage the 

 germ glands are similar to those of the preceding stage. In one 

 (larva no. 622) a large number of germ cells were in mitosis. The 

 products of the division of an isolated germ cell may become 

 enclosed, each in its own follicle, or they may remain together 

 as a part of a nest of cells enclosed in a common follicle. The 

 follicle cells of such a nest often stretch into the spaces between 

 the germ cells, while other folhcle cells, recognizable by the form 

 and structure of their nuclei, are detached from the follicular mem- 

 brane and lie free among the enclosed germ cells. Follicle cells 

 occur in all stages of the process of detaching themselves from 

 the foUicles and also from the peritoneum, and of penetrating 

 between the germ cells of the nests (fig. 40). In this way, as 

 the germ cells increase in number, old cell nests are broken up 

 and new ones formed, so that in this larva very few nests remain 

 which have more than eight cells. The many follicle cells neces- 

 sary to enclose the increasing number of nests are probably pro- 

 duced by proliferation of cells already in the follicles as well as 

 of those in the peritoneal epithelium. If this be so, the mitoses 

 must take place very rapidly, for dividing nuclei in either fol- 



