3o8 



EVOLUTION, GENETICS, AND EUGENICS 



in which a single large, primordial germ-cell {p. g. c.) has already been 

 set apart at one end of the developing embryo. The nuclei of the rest 

 of the embryo still lie in a continuous protoplasmic mass which has 

 not yet divided up into separate cells. The densely stained nuclei at 

 the opposite end of the section are the remnants of nurse-cells which 

 originally nourished the egg. Fig. 45, B, is a longitudinal section 



o3g 



Fig. 45. — A, germ-cell (p.g.c.) set apart in the eight-celled stage of cleavage 

 in Miastor americana. {After Hegncr.) The walls of the remaining seven somatic 

 cells have not yet formed, though the resting or the dividing {M p) nuclei may be 

 seen; c R, chromatin fragments cast off from the somatic cells; B, section length- 

 wise of a later embryo of Miastor; the primordial egg-cells {oog^) are conspicuous. 

 {From Guyer, after Hegner.) 



through a later stage in the development of Miastor; the primitive 

 germ-cells {oog) are plainly visible. Still other striking examples 

 might be cited. Even in vertebrates the germ-cells may often be 

 detected at a very early period. 



Significance of the early setting apart of the germ-cells. — ^It is of 

 great importance for the reader to grasp the significance of this early 

 setting apart of the germ-cells because so much in our future discussion 

 hinges on this fact. The truth of the statement made in a previous 

 chapter that the body of an individual and the reproductive substance 



