228 FOUNDATIONS OF BIOLOGY 



the chromosomes as the chief factors in the transmission of 

 characters from cell to cell, and therefore in inheritance. 



2. Gametes 



Returning now to the origin of gametes. The spermato- 

 gonia and oogonia in the reproductive organs are, together 

 with all the cells forming the body proper, direct descendants 

 by mitotic cell division from the fertilized egg which gave rise 

 to the individual organism. This is equally true of the chro- 

 mosomes themselves and accordingly every cell of the animal 

 has the same number of chromosomes as the fertilized egg. 



Fertilization, as we now know, always consists of the 

 fusion of two gametes, whether it is in plants or animals; a 

 fusion of nucleus with nucleus and cytoplasm with cytoplasm 

 to form a zygote, which therefore is one cell reconstructed from 

 two. Such being the case, one of two things must happen at 

 fertilization. Either the fertilized egg must have double the 

 chromosome number, that is a set contributed by both egg 

 arid sperm; or some method must exist by which the chromo- 

 somes of the gametes are reduced in number to one half that 

 characteristic of the somatic cells. 



As a matter of fact a reduction in the number of chromo- 

 somes always takes place sometime during the life history. 

 In plants such as the Mosses, Ferns, and Flowering Plants, it 

 occurs at the formation of the spores. Thus it follows that 

 the gametophyte contains half as many chromosomes as the 

 sporophyte, and the sporophyte number is restored by the 

 union of the gametes. It must be borne in mind, however, 

 that the familiar plants are sporophytes which, for all prac- 

 tical purposes, directly produce sporophytes because the 

 gametophyte is reduced almost to the vanishing point. The 

 chromosome number of the parent sporophyte and the 

 sporophyte in the seed is the same. But we cannot digress 



