36 GERM-CELL CYCLE IN ANIMALS 



of the nurse cells and follicle cells is important since 

 in a few cases the germ cells themselves are known 

 to give rise to them. There is thus a second differ- 

 entiation whereby somatic cells (follicle cells or 

 nurse cells) become differentiated from germ cells 

 (oogonia or spermatogonia). In some species, such 

 as Miastor, we can prove without question that both 

 the nurse cells and follicle cells are of mesodermal 

 origin, and that the germ cells give rise only to germ 

 cells. On the other hand, there are instances in 

 both vertebrates and invertebrates of a common 

 origin of germ cells and somatic cells from oogonia 

 and spermatogonia. Perhaps the most striking 

 examples are the differentiation of the nurse cells 

 and ultimate oogonia in the water beetle, Dytiscus, 

 and the differentiation of the Sertoli cells and ulti- 

 mate spermatogonia in man. (See Chapter V.) 

 Haecker (1912) distinguishes' between a somato-ger- 

 minative period and a true germinative period ; the 

 former is that during which the primordial germ cells 

 are established and the latter that of the differentia- 

 tion of nurse cells and ova. 



6. The Growth Period. The last divisions 

 of the oogonia and spermatogonia are followed by 

 the growth of these cells. The extent of this growth 

 depends, in the case of the female, upon whether 

 or not the mature egg is to be supplied with an 

 abundance of nutritive material. Nurse cells, fol- 

 licle cells, and circulating fluids may all assist in the 

 enlargement of the oogonia. If the eggs are small, 

 sufficient nutriment is supplied by surrounding 



