MATURATION DIVISIONS OF THE EGG 209 



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

 each of which may fertilize an egg. In each case the primordial 

 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 chromosomes as are formed at ordi- 

 nary vegetative divisions (in modern terminology this is called 

 the haploid number). Each chromosome, however, is double 

 consisting of two chromosomes lying side by side or end to end 

 (Fig. 90) . Reduction therefore, at this stage is not real hence the 

 phrase pseudo-reduction is applied to it. The two parallel 

 parts of each chromosome then divide longitudinally and the 

 entire chromosome frequently contracts into a smaller four- 

 parted chromosome termed a tetrad (Fig. 90). A mitotic 

 figure is then formed which migrates toward the periphery of 

 the egg and the nucleus divides equally, one-half of v 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 chro- 

 mosomes 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 has 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, and in some cases the 

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

 entrance of the sperm. 



