36 THE BREEDING OF ANIMALS 



the case. After the reduction of the chromosomes in 

 the maturation of the germ-cells and accompanying the 

 formation of the polar bodies, the number of chromosomes 

 is reduced to one-half the number existing in the body- 

 cells and these uniting with an equal number from the 

 sperm constitute the normal number characteristic of 

 the tissue cells. As Wilson says, " We have thus what 

 must be reckoned as more than a possibility that every 

 cell in the body of the child may receive from each parent 

 not only half of its chromatin substance, but one-half 

 of its chromosomes, as distinct and individual descendants 

 of those parents." 



35. Results of union of egg and sperm. Extraor- 

 dinary changes follow immediately the physical union 

 of the egg and spermatozoon. These changes are of 

 the most fundamental significance in connection with 

 discussions of development and inheritance. 



36. Changes in the ovum. The entrance of the 

 sperm seems to exert an influence which permeates the 

 entire constitution of the egg substance. Various and 

 important changes now take place, ending finally in an 

 exact union of the germ substance of the two sex cells, 

 thus forming the new or daughter cell. The daughter 

 cell is the beginning of a new individual and becomes the 

 offspring of the parents, from which the sperm and ovum 

 were derived. When the sperm enters the egg, the 

 vitelline membrane is thrown around the outside of the 

 ovum. The nucleus of the egg, which is now called the 

 germinal vesicle, moves to the wall, and changes occur 

 which result in the formation of the polar bodies. This 

 process prepares the germ nucleus of the egg for ferti- 

 lization. 



The polar bodies are formed by successive divisions 



