128 DOMESTICATED ANIMALS AND PLANTS 



probably man it is 16 ; in ascaris, 4 or possibly 2 ; while in 

 artemia it is 168. 



There is still another peculiarity about the chromosomes, 

 namely, that the nuclei from the sex cells have but half tJie 

 usual number; but after union of the two nuclei from the sepa- 

 rate parents, the full number is restored, and from then on cell 

 division and growth begin and proceed in the usual way, barring 

 accident, till full maturity is attained. 



These chromosomes, therefore, appear of importance, not 

 only in growth (for the operation of cell division seems to be 

 preceded if not characterized by the division of the chromo- 

 somes), but they appear to be par excellence the hereditary sub- 

 stance, that is, the bearers of heredity ; all of which encourages 

 the belief that most characters are in some way identified with 

 definite portions of the hereditary matter of the nucleus, that is, 

 with its chromosomes. 



Development, or growth and differentiation. The process by 

 which these two nuclei after fusion succeed in producing a 

 new individual combines two phenomena, namely, growth and 

 differentiation. 



The process of growth means that this new cell, which is 

 made up of two others, is able now to absorb food materials and 

 to first increase in size, then to multiply in numbers by the 

 process of repeated and indefinite division, until what was once 

 a single cell comes to be a new individual with thousands of cells. 



This is growth and it is astonishing enough, but the chief 

 marvel is the differentiation that attends it. When these thousands 

 of cells have developed from the one original, it will be found 

 that they are not all alike ; some are stem, others root, still others 

 leaf, flower, or fruit. In the case of animals some are muscle 

 cells, others constitute liver, brain and nerve, arm, leg, or eye. 



In other words, growth has been attended by differentiation, 

 so that the single minute mother cell has been able to give rise 

 to many cells of many kinds, only a few of which are able to 

 repeat the process of reproduction. Not only has differentiation 



