430 THE MAINTENANCE OF SPECIES 



other cells. Each of the daughter cells is now a secondary spermatocyte 

 producing two spermatids by the next division in which each of the 

 remaining chromosomes, as in usual mitosis, splits longitudinally in 

 half, thus maintaining the haploid number in each cell. Each sperma- 

 tid eventually undergoes a metamorphosis into an active sperm with- 

 out further cell divisions. Thus, each primary spermatocyte pro- 

 duces four functional sperm. 



Formation of Ova — Oogenesis 



Oogenesis differs from spermatogenesis only in certain essential 

 respects, although the corresponding stages must necessarily be 

 designated differently. Thus the primary germ cells produce oogonia 

 which in turn produce primary and secondary oocytes, polar bodies, and 

 finally ova. In the period of growth intervening between the oogonium 

 and its transformation into a primary oocyte there is a large accumu- 

 lation of stored food and an accompanying increase in size. In the 

 next stage, when the primary oocyte undergoes its reduction division 

 the resulting cells are of unequal size, one becoming much larger 

 than the other, having monopolized all of the yolk. The smaller 

 one is in reality an aborted secondary oocyte and is called the first 

 polar body. The second maturation division again results in the 

 formation of a relatively large egg and a tiny second polar body. 

 Sometimes the first polar body likewise undergoes division, formmg 

 a total of three small polar bodies and one large ovum. 



The process of fertilization brings together the male and the female 

 pronuclei, each of which contains the haploid number of chromosomes. 

 By this means the diploid number, or full complement of chromosomes, 

 is restored. Each chromosome, moreover, is composed of a number 

 of genes arranged on it like a string of beads. The manner in which 

 this mechanism functions in bringing about variations in the offspring 

 will be considered in the unit on genetics (page 457) . 



The New E)mbryology 



The question as to just how far back one can trace the develop- 

 mental pattern of an embryo is one which has long fascinated the 

 zoologist. Great strides along this line have been made in recent 

 years by the students of experimental embryology. We know that 

 fertilized ova develop with great rapidity into well-formed embryos, 

 characterized first by germ layers, later by tissues, and finally by 



