5o6 EVOLUTION, HEREDITY, EUGENICS 



single components, called monads, just as in the development of the 

 spermatid. 



During the first maturation division, the spindle appears near 

 the periphery, and the nuclear material is divided equally, but the 

 cytoplasmic material most unequally, so that the resultant budded 

 ''first polar body " is extremely small. It may later divide, but 

 becomes degenerate and of no significance. The larger cell is called 

 the secondary oocyte. 



In the second maturation division, the members of the dyads of 

 the secondary oocyte are separated but the cytoplasm is again 

 unequally distributed and the second polar body is given off. The 

 larger cell undergoes no further changes (in contrast to the sperma- 

 tid) but becomes the ^nature egg. Just as in the spermatozoa, we 

 find that the eggs contain one-half the number of chromosomes 

 present in the oogonia.^ 



Fertilization. — In many of the lower animals we find that fertili- 

 zation takes place after the eggs are laid, but in higher forms, such 

 as reptiles, birds, and mammals, it occurs in the oviduct of the fe- 

 male. 



In some forms the spermatozoan enters the egg before the polar 

 bodies are formed, in others it may enter after the first polar body 

 is formed, but before the second has been produced, and in still 

 others, maturation of the egg has been completed before a sperma- 

 tozoan is admitted. Polyspermy is not unknown, but in most 

 animals only a single spermatozoan penetrates the egg. 



The spermatozoan forms a jjiale pronucleus, which unites with 

 the female pronucleus, and subsequent divisions are by ordinary 

 mitosis. 



Two species of fishes, having different shaped chromosomes, were 

 hybridized by Dr. Moenkhaus, who showed that there can be no 

 question that subsequent multiplication of the cells of the fertilized 

 ovum is accompanied by the equal distribution of chromatin from 

 each parent. 



The chromosomes split longitudinally into two parts, separate, 

 and group themselves in two clumps around the poles of a delicate 

 spindle; then the cell divides. Thus by mitotic division beginning 



* Carothers, E. E. 1926. The Maturation Divisions in Relation to the Segrega- 

 tion of Homologous Chromosomes. Quart. Rev. of Biol., vol. i, pp. 4I9-435. The 

 sections on mitosis and gametogenesis were corrected by Dr. Carothers. 



