294 EVOLUTION AND GENETICS 



essential features is apparently always the same. Omitting con- 

 sideration of the chromosomes associated with sex, we find that 

 the fully developed primary spermatocyte or oocyte contains as 

 many pairs of chromosomes as there are kinds. In preparation 

 for the two maturation divisions which follow close upon synapsis, 

 each of the chromosomes has split longitudinally, so that each 

 synaptic pair is in reality a group of four halves of chromosomes 

 called a tetrad. Tetrads are not represented in Figure 169, which 

 is designed to show in the simplest possible way the fundamental 

 features of the process. In Figure 170E is shown a primary oocyte 

 of Ascaris megalocephala containing two tetrads. The diploid 

 number of chromosomes in this species is four. 



Maturation Divisions. Closely following the formation of tet- 

 rads each primary cell undergoes two successive divisions which 

 distribute the four parts of each tetrad among the four resulting 

 daughter cells. One of these divisions separates the two synaptic 

 mates, or entire chromosomes, and produces daughter cells with 

 only the haploid number. This is called the reduction division. 

 The other merely separates halves of chromosomes, so that 

 daughter cells and parent cell are similar. It is called the equation 

 division (Fig. 169C-E). The order in which the two divisions 

 occur may vary among different species. 



The cells derived from primary spermatocytes are called second- 

 ary spermatocytes, and those of the next generation spermatids. 

 Spermatids undergo a process of differentiation and become motile 

 spermatozoa. 



The maturation divisions in the female differ in that only one 

 primary and one secondary oocyte are produced. Each of the 

 maturation divisions results in the concentration of cytoplasm in 

 this one cell, while the necessary chromosome reduction is accom- 

 plished l)y the formation of abortive polar bodies. The first polar 

 body (Fig. 169D) is similar in nuclear organization to the secondary 

 oocyte and the second is similar to the mature ovum, with the 

 exception of such qualitative differences as may result from reduc- 

 tion. The first polar body sometimes divides as shown in Figure 

 169E. 



The Mature Germ Cells. Although the mature germ cells 

 of both sexes contain the haploid number of chromosomes they 

 may be very different in other ways. The ovum is a large cell 

 containing an abundance of cytoplasm and often a large quantity 



