THE LIVING MACHINE BUILDING FACTORS. 153 



the result of such division should possess similar 

 characters so long as this method of division con- 

 tinues. But after a little in the development of 

 the egg a differentiation among the daughter cells 

 arises. They begin to acquire different shapes and 

 different functions. This we can only believe to 

 be the result of a differentiation in their chro- 

 matin material. In the cell division the chromo- 

 somes no longer split into equivalent halves, but 

 some characters are portioned off to some cells 

 and others to other cells. Those cells which are 

 to carry on digestive functions when they are 

 formed receive chromatin material which especially 

 controls them in the performance of this diges- 

 tive function, while those which are to produce 

 sensory organs receive a different portion of the 

 chromatin material. Thus the adult individual 

 is built up as the cells receive different portions 

 of this hereditary substance contained in the 

 original chromosomes. The original chromosomes 

 contained all hereditary characters, but as devel- 

 opment proceeds these are gradually portioned 

 out among the daughter cells until the adult is 

 formed. 



From this method of division it will be seen 

 that each cell of the adult does not contain all 

 the characters concealed in the original chro- 

 mosomes of the egg, although each contains a 

 part which may have been derived from each 

 parent. It is thought, however, that a part of 

 the original chromatin material does not thus 

 become differentiated, but remains entirely un- 

 changed as the individual is developing. This 

 chromatin material may increase in amount by 

 assimilation, but it remains unchanged during the 

 entire growth of the individual. It thus follows 



