34 GERM-CELL CYCLE IN ANIMALS 



coelomic sacs ; each group probably contains an 

 equal number of cells. The smallest number I 

 have counted in one group at this time is thirty; 

 the largest number, thirty-four. As there is some 

 difficulty in obtaining an accurate count, it seems 

 probable that the sixty-four germ cells are equally 

 divided and that each germ gland receives thirty-two. 

 Some of the germ cells migrate not only laterally 

 along the germ gland but also back toward the 

 posterior end of the egg, where we find them forming 

 narrow strands in the last abdominal segments. 

 From this stage on, the germ cells are not very active ; 

 they move closer to one another to form the compact 

 germ glands. I was unable to determine whether 

 the later movements of the germ cells are due to 

 an active migration or to the tensions created by 

 the growth of the surrounding tissues; the latter 

 seems the more probable" (Hegner, 1909a, p. 280). 



It is thus evident that during the blastoderm stage 

 the germ cells of this beetle are actually outside of the 

 egg. How well this illustrates the theory of primary 

 cellular differentiation, that is, the differentiation of 

 germ cells from somatic cells, since the two sorts 

 are here completely separated, the former constitut- 

 mg a group in contact with but not connected with 

 the somatic cells. Later, as the germinal con- 

 tinuity hypothesis demands, the germ cells migrate 

 into the embryo, there to be nourished, transported, 

 and protected by the body until they are ready to 

 separate from the somatic cells, and thus to give rise 

 to a new generation. 



