38 



BIOLOGY OF DEATH 



the former are borne in the female, and the latter in the 

 male body. Both sorts undergo a complicated prepara- 

 tion for union, the result of which is that when union does 

 occur each party to it contributes either an exactly equal 

 or an approximately equal amount of hereditary mate- 





FIG. 9. Section across the posterior part of an embryo dog-fish (acanthias) of 3.5 mm., to 

 show the compact cluster of germ cells on one side. The germ cells in later stages migrate 

 from this primitive position, moving singly or in small groups. Ect, ectoderm: Md, medullary 

 canal or primitive spinal cord; Nch, notqchord; Mes, mesoderm: Ent, entoderm: X, cellular 

 strand connecting the germ cell cluster with the yolk. (From Minot after Woods, with the 

 permission of the publishers, G. P. Putnam's Sons). 



rial. After union has taken place the fertilized ovum 

 or zygote presently begins to divide, first into two cells, 

 these again to four and so on, until by a continuation of 

 this process of division with concomitant differentiation 

 the whole body is formed. As the animal develops by 

 repeated cell division and differentiation, it is frequently 

 found that at the very early stage the cells which are to 

 be the germ cells of the next generation are clearly re- 



