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 
(odgonia 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 odgonia 
and spermatogonia. Perhaps the most striking 
examples are the differentiation of the nurse cells 
and ultimate odgonia 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. Tue GrowtH Prriop. The last divisions 
of the odgonia 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 odgonia. If the eggs are small, 
sufficient nutriment is supplied by surrounding 
