CHARACTERISTICS: RELATION TO ENVIRONMENT 233 



Relation of Gene Action to Internal Environment 

 in Individual Development 



The organism starts as a single cell, the fertilized egg cell. 

 This contains the great number of genes, a thousand or 

 so, grouped together in the visible chromosomes. 



These chromosomes are seen to go at once to work. They 

 are imbedded in a mass of material, the cytoplasm, form- 

 ing the body of the cell. They take up material from the 

 cytoplasm, so that they swell, enlarge, become vesicles, be- 

 come crowded together to form what we call the nucleus 

 (see figure 15, Chapter II). They chemically change this 

 cytoplasm; they give it off again into the cell, so that the 

 chromosomes are again minute condensed bodies. The 

 changed cytoplasm that they have given off into the cells 

 alters the nature and structure of the cell, often in a 

 strongly marked, visible way. 



This process of changing the cytoplasm by the action of 

 the genes is the fundamental thing in development. The 

 genes repeat this process over and over again — taking in 

 cytoplasm, modifying it, giving it off in changed condition 

 — leaving the genes themselves unaltered. 



While this is going on, the cell divides into 2, 4, 8 cells, 

 and so on. The different cells receive diverse kinds of the 

 cytoplasm that has been worked over by the genes. So the 

 different cells gradually become diverse. As the cells divide, 

 each chromosome divides, each gene divides, and half of 

 every gene goes into each cell, where it grows again to the 

 original size. Thus every cell of the body contains all the 

 genes, the complete set. The cells differ in their cytoplasm; 

 they do not differ, as a rule, in their genes. 



The cells keep getting diverse, in this way, till some 

 produce bones, some muscles, some nerves, some eyes, some 

 hands, till finally we have the complex individual with all his 

 parts and functions. 



