214 THE PERPETUATION OF ADAPTATIONS 



zoon, on the other hand, four functional spermatozoa are formed, 

 each of which may fertilize an egg. In each case, the primor- 

 dial germ cells of the germinal endothelium are similar; each 

 has the number of chromosomes characteristic of the species 

 (in modern terminology, the diploid number), but the egg- 

 forming cells, at an early period, begin to enlarge and to deposit 

 stores of yolk in the cell body. The chromatin of the nucleus 

 collects in a thick fibrous mass on one side of the nucleus 

 (synapsis stage), and from it emerge one-half as many chromo- 

 somes as are formed at ordinary vegetative divisions (in modern 

 terminology, this is called the haploid number) . Each chromo- 

 some, however, is double, consisting of two chromosomes 

 lying side by side or end to end (Fig. 90) . Reduction, therefore, 

 at this stage has not actually taken place, hence the phrase 

 pseudo-reduction is applied to it. The two parallel parts of 

 each chromosome then divide longitudinally, and the entire 

 chromosome, in many cases, contracts into a smaller four- 

 parted chromosome termed a tetrad (Fig. 90). A mi to tic 

 figure is then formed, which migrates toward the periphery of 

 the egg, and the nucleus divides equally, one-half of each tetrad 

 passing into a daughter nucleus. While the nucleus divides 

 thus equally, the egg cell divides unequally; only enough egg 

 protoplasm is divided off to surround the one daughter nucleus. 

 This becomes pinched off at the surface of the egg to form a 

 minute bud-like cell termed the polar body. Both nuclei 

 then pass directly into a second division phase, the chromo- 

 somes undergoing no further change. By this second divi- 

 sion, each remaining half tetrad (now called a dyad) is separated 

 into its two component parts, one going to each daughter 

 nucleus, and a second polar body is formed from the egg. The 

 first polar body meanwhile may have divided to form two small 

 cells, which with the second polar body and the functional egg 

 make up the four cells derived from the primordial germ cell. 

 In many cases the first polar body does not divide, and in some 

 cases these maturation divisions do not take place until after 

 the spermatozoon has entered the egg; sometimes the first 

 polar body is formed before, the second polar body after, 

 entrance of the sperm. 



