670 METABOLISM 



is again combined with some simple protein to form miclein, the essen- 

 tial constituent of the chromatin of the nucleus. One of the oxypurines, 

 hypoxanthine, may also exist combined with phosphoric acid and carbo- 

 hydrate to form a substance present in muscle and known as inosinic 

 acid. 



The simplest form of nucleic acid is that known as guanylic, which is 

 found in certain organs (liver, pancreas, etc.) side by side with the more 

 complex variety. It consists of phosphoric acid, a pentose (5 C- atom 

 sugar) and the amino purine, guanine. The pentose which can be 

 detected in these organs is apparently derived solely from guanylic 

 acid. The more complex form of nucleic acid, and probably that present 

 in all nuclei, is composed of phosphoric acid, a hexose (in animal cells) 

 or a pentose (in vegetable cells), the two amino purines, adenine and 

 guanine, and the two pyrimidine bases, cytosine and thymine. 



Nucleic acid may therefore ~be considered as a compound of polyphos- 

 phoric acid, containing carbohydrate groups, which serve to link the phos- 

 phoric acid molecules to those of purine and pyrimidine. 



It has been found necessary to introduce certain terms to designate 

 the different parts of the nucleic acid molecule ; thus, the whole molecule 

 is called a tetranucleotide, composed of four mononucleotide molecules, 

 each of which consists of a phosphoric acid molecule plus a nucleoside, 

 which again is composed of a purine or pyrimidine nucleus attached to 

 pentose or hexose. The nucleoside is so named because it is similar in 

 structure to a glucoside. 



Apart from differences in the carbohydrate group, it appears that 

 there is a close similarity in the structures of nucleic acids from dif- 

 ferent cells. This would indicate a common function for them all, which 

 may be either of a structural or of a physiological nature ; that is, nucleic 

 acid may have to do with the sustentacular material that builds the 

 nucleus, or it may have to do with some physiological function common 

 to all cells, such as irritability, or growth, or respiration. If nucleic 

 acid is merely a sustentacular material, then the study of the behavior 

 of chromosomes and chromatine in cells can not have the significance 

 that it would have were nucleic acid concerned in the more vital activ- 

 ities of the nucleus. All the so-called nuclear stains owe their specific 

 staining properties to the fact that they are of a basic nature and com- 

 bine with nucleic acid. Until we know more definitely what the exact 

 function of nucleic acid may be, it is unwise to place too much weight 

 on the behavior of the chromosomes in cytologic researches. 



By studying the behavior of cells (ameba) from which the nucleus 

 was removed, it has been found that the process of reproduction and 

 growth alone are affected. The other functions proceed normally. In 



