THE CELL AND PROTOPLASM. 131 



From now the divisions of the egg follow rapidly 

 by the normal process of cell division until from 

 this one egg cell there are eventually derived 

 hundreds of thousands of cells which are gradu- 

 ally moulded into the adult. All of these cells 

 will, of course, contain four chromosomes ; and, 

 what is more important, half of the chromo- 

 somes will have been derived directly from the 

 male and half from the female parent. Even 

 into adult life, therefore, the cells of the animal 

 probably contain chromatin derived by direct 

 descent from each of its parents. 



THE SIGNIFICANCE OF FERTILIZATION. 



From this process of fertilization a number of 

 conclusions, highly important for our purpose, 

 can be drawn. In the first place, it is evident 

 that the chromosomes form the part of the cell 

 which contain the hereditary traits handed 

 down from parent to child. This follows from 

 the fact that the chromosomes are the only 

 part of the cell which, in the fertilized egg, is 

 derived from both parents. Now the offspring 

 can certainly inherit from each parent, and 

 hence the hereditary traits must be associated 

 with some part of the cell which is derived from 

 both. But the egg substance is derived from 

 the mother alone; the centrosome, at least in 

 some cases and perhaps in all, is derived only 

 from the father, while the chromosomes are 

 derived from both parents. Hence it follows 

 that the hereditary traits must be particularly 

 associated with the centrosomes. 



