2o6 ESSENTIALS OF BIOLOGY 



but the chemical reactions of which do not altogether support this 



formula. 



It is the simplest possible chemical body which has all the 



characteristic organic elements, C, N, H, O. It is a body of 



profound metabolic importance ; curiously versatile in its chemical 



behaviour ; now readily undergoing disintegrations and again 



undergoing synthesis. It is obviously of the highest possible 



significance in the processes of animal metabolism. Now in the 



bases that make up the nucleotides, that is, in thymine, guanine, 



adenine, cytosine, uracil, etc., there are the purine and pyrimidine 



nuclei, or chemical foundations. These are all to be derived from 



/N = 

 the urea-skeleton = C\ 



^N = 



It is difficult to consider this very peculiar chemical structure 

 of nuclein without coming to the conclusion that it is, so to speak, 

 the armament of rapid metabolic changes in the nucleus and its 

 immediate cytoplasmic environment. The protamine part of 

 the chromatin is probably chemically inactive : it is a stable 

 substance, strongly basic, not digested by pepsin but broken down 

 by trypsin. The nucleic acid, on the other hand, is easily dis- 

 integrated. Nucleases, or enzymes peculiar to nuclear structures, 

 easily break it down. The carbohydrates in the molecule are 

 very reactive when broken off from the nitrogenous bases. They 

 may be oxidized, when much available energy is set free, or they 

 may even afford oxygen to other systems. (Note, in this connec- 

 tion, the anaerobic respiration of some cells by the disintegration 

 of sugars.) The nitrogenous bases, when they are thus broken 

 away from the nuclein molecule give the necessary materials for 

 other syntheses. What we see in the nucleus is, for one part, 

 obviously the means for rapid and versatile chemical reactions 

 on the minute scale. Possibly the segregations of strongly reactive 

 substances in multitudes of minute " vessels," the nuclei, in close 

 proximity to each other yet always quite separable, is a condition 

 of some significance. 



We shall return to this matter of the chemistry of the cell 

 nucleus in a later chapter. We have now to consider what is the 

 nature of the developmental process. 



