2o6 



EVOLUTION, GENETICS, AND EUGENICS 



clear protoplasm at the animal pole, are not in a position to bring about 

 an equal cell division. Hence the maturation divisions are very un- 

 equal (Fig. 48). The first maturation division results in a large cell, 

 not appreciably smaller than the full-grown egg, and a very tiny cell, 

 called the first polar body. The second maturation division results in 

 a second polar body and the mature female gamete, or unfertilized egg. 

 Sometimes the first polar body divides again, sometimes not. Typi- 

 cally, however, four female gametes are produced, corresponding in 



Fig. 48. — Diagram to illustrate oogenesis, a, showing the diploid number of 

 chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and in 

 oogonia; b, the pairing of corresponding mates preparatory to reduction; c, d, the 

 reduction division, giving off the first polar body; e, egg preparing to give off the 

 second polar body, first polar body ready for division; /, second polar body ready 

 for division; g, second polar body given off, division of first polar body completed. 

 The egg nucleus, now known as the female pronucleus, and each polar body contain 

 the reduced or haploid number of chromosomes. (From Guyer.) 



number to male gametes similarly produced; but three out of every 

 four eggs, namely, the polar bodies, are abortive and die because of 

 deficiency of cytoplasm, leaving only the one large well-nourished egg 

 as the progeny of each primordial germ cell that completed the period 

 of growth. Synapsis and the reduction division are the same in 

 oogenesis as in spermatogenesis (Fig. 49). 



Now, during both spermatogenesis and oogenesis the united pairs 

 of homologous chromosomes, one derived from the father and one from 

 the mother of the previous generation, arrange themselves during the 

 reduction division quite independently of one another, so that some 

 maternal and some paternal chromosomes enter each gamete. Where 



