68 PETER OKKELBERG 



deg.g. in the figure. The cytoplasm disintegrates and finally the 

 chromatin masses break up and the fragments are scattered 

 through the whole cyst. Such fragments are shown at g.f. 

 Other stages occur showing the various steps up to the final 

 dissolution of the cysts. No large cysts containing cells in synap- 

 sis have been found to break up by the inward migration of fol- 

 licle cells. The final fate of all of them and of their enclosed 

 cells is degeneration. Rarely, however, a single cell of such a 

 cyst may enter the growth phase. A cyst containing such a 

 cell is shown in figure 64. This contains a large oocyte among 

 the smaller cells of the cyst. Whether such oocytes continue to 

 grow and form functional ova is not known, but it is certain 

 that nearly all of the growing oocytes are derived from isolated 

 germ cells or from the breaking up of small cysts. Other sec- 

 tions, showing both cysts and oocytes in about equal numbers, 

 are given in figures 70 and 71 from larvae 55 mm. and 62 mm. 

 long, respectively. 



In the middle of the period of sex differentiation, therefore, 

 there are in the same gland undifferentiated germ cells that are 

 still dividing; germ cells in the various prophases of heterotypic 

 division; cells that have entered the period of growth, forming 

 recognizable oocytes, and cells undergoing degenerative changes. 



The gland described above represents only one type of germ 

 gland during this period — a type in which the number of growing 

 oocytes and cysts is approximately the same. There are other 

 glands with very few growing oocytes, and still others in which 

 there are very few cysts. Figure 72 is a section through the 

 germ gland of a larva 50 mm. long in which there are very few 

 growing oocytes, and none of these are shown in the section. 

 The gland is filled with cysts, some of which contain undifferen- 

 tiated germ cells, while some contain cells in the synaptic phases. 

 It is often found that if a gland seems free from growing oocytes 

 when examined superficially, more careful search usually reveals 

 at least a few of them scattered about through the gland. In one 

 such gland only one growing oocyte could be found. 



Figure 73 is a section of the germ gland of a larva 71 mm. long. 

 In this larva there are only a few growing oocytes, only one of 



