428 THE MAINTENANCE OF SPECIES 



umbilical cord to the embryo. From the surface of the chorion 

 fingerhke projections, or villi, push out which interdigitate with 

 similar fingerhke processes of the uterine wall, thus facilitating the 

 maintenance of metabolism. This portion of the chorion together 

 with the wall of the uterus in which the embryo is embedded is usually 

 designated as the placenta. While there is no exchange of blood 

 between the parent and embryo, their two blood streams in the case 

 of the primates are separated only by the lining of the fetal capilla- 

 ries, the connective tissue surrounding them, and the epithelial layer 

 on the surface of the chorionic villi. While the allantois does develop 

 in the mammalian embryo, it is incorporated into the growing placenta 

 and in primates is really functionless, except for the proximal portion 

 which is transformed into the urinary bladder of mammals. As the 

 embryonic membranes are not permanent structures they are dis- 

 carded at birth. 



Elaboration of Germ Cells, or Gametogenesis 



It should be borne in mind that the germ cells themselves can be 

 traced back in the developing embryo only to a certain point which 

 varies in different groups. In the chick, for example, the germ cells 

 may be traced to the anterior margin of the blastoderm. In some 

 invertebrates, such as Ascaris megaloccphala hivalvens, it has been 

 shown that the germ cells may be detected at the thirty-two cell stage. 

 In the latter instance the primordial germ cell may be readily detected 

 by its size. 



While the primordial germ cells are present early in the life of the 

 individual, it frequently happens that the organism does not mature 

 for some time and consequently the development, or maturation, of 

 functional germ cells is delayed. Usually the maturation process 

 covers a considerable period of time which, in the case of a male, 

 terminates in the elaboration of sperm. Hence the entire process is 

 called spermatogenesis, while in the female the production of ova is 

 known as oogenesis. Both phenomena may be spoken of collectively 

 as gametogenesis. 



Formation of Sperm — Spermatogenesis 



The primordial germ cells of the male undergo an extended period 

 of division, the resulting cells of which are designated as sperma- 

 togonia. These reproduce other spermatogonial cells by normal 



