REPRODUCTION 35 



primarily concerned with the nuclei of the egg and the 

 sperm. 



In most species of animals, there is a definite attrac- 

 tion existing between the egg and the sperm-cell, which 

 causes the spermatozoon to attach itself to and finally 

 penetrate the egg. This attraction is probably of a 

 chemical nature. Pfeffer found that solutions of malic 

 acid were as successful in attracting the sperm-cells of 

 ferns as the substance which was thrown off by the female 

 sex cells. In other experiments, other chemical sub- 

 stances have exhibited a specific attraction for the sperm. 

 The seat of this chemical substance which pulls the 

 sperm to the egg seems to be located in the cytoplasm of 

 the egg and not in the nucleus. 



The point at which the spermatozoon enters the egg 

 is often predetermined by the existence of a depression 

 or opening (micropyle) in the wall of the ovum. In 

 some cases, there exists a special protoplasmic attraction 

 cone at which point the sperm enters the egg. The 

 entrance of the sperm in most cases is followed rapidly 

 by the formation of a vitelline membrane which surrounds 

 the egg and prevents the entrance of other spermatozoa. 



Normally in mammals one sperm only enters the egg. 

 When through accident two or more spermatozoa enter 

 the substance of the egg, the developmental changes are 

 abnormal and the daughter nucleus soon dies. 



34. The chromosomes. When the germ nuclei unite 

 to form the daughter nucleus of the new cell, it seems 

 probable now that the chromatin substance does not fuse 

 in true fashion but the chromosomes may lie side by side 

 within the nucleus of the new cell. It is not definitely 

 determined that these chromosomes remain thus separate 

 and apart throughout the life of the cell, but such may be 



