300 



REPRODUCTION IN ANIMALS 



spring inherits equally from either parent. The new 

 individual is certain to be slightly different from either 

 parent or their ancestors and there is a chance that it 

 may be better fitted to continue the race. (See Chap. 

 XXXV.) 



Individualiti' of the Chromosomes. — It has been 

 stated in the preceding paragraph that all of the body- 

 cells and the reproductive elements, throughout the 

 greater part of their development, have a definite number 

 of chromosomes. And this number is established at the 



Fio. 71. — Diagram illustrating the fertilization of the egg and 

 early development. A, entrance of the sperm. B, approach of 

 sperm to egg nucleus. C, union of nuclei. D, splitting of chromo- 

 somes (those of sperm solid black). E, separation of the chromo- 

 somes prior to F, the divi.sion of the embryo into two cells. 



time of fertilization by the union of the egg and sperm 

 nucleus. Some of the insects, for example, have eight 

 chromosomes, certain sea-urchins sixteen, some of the 

 worms twenty- four, while there are forty-eight in man. 

 If there are eight chromosomes in the fertilized egg, this 

 characteristic number can be counted at each of the thou- 

 sands of succeeding divisions. Indeed, in some species 



