308 THE PHYSIOLOGY OF REPRODUCTION 



formed from them in the process of hydrolysis by a secondary 

 reaction. 



4. Derivatives of pyrimidine 



(1) N=CH (6) 



I I 



(2) HC CH (5) 



(3) N CH (4) 



(2) HC CH (5) 



UH 

 namely 



Cytosine = 6 Aiuino-2 Oxypyrimidine ; 



Uracil = 2-6-Dioxypyrimidine ; 

 Thymine = 5 Methyl-2-6 Dioxypyrimidine (Methyl-Uracil). 



Of these cytosine and thymine are present as such in the nucleic 

 acids obtained from animals, while uracil is formed from cytosine by 

 a secondary reaction in the process of the splitting up of the nucleic 

 acid. 



In the nucleic acids of plant origin uracil is bound iip as such and 

 takes the place of thymine. So far as our present knowledge goes 

 only two nucleic acids or groups of nucleic acids occur in nature ; 

 one obtainable from the nuclei of animal cells, the other from the 

 nuclei of plant cells. The difference consists, as has just been stated, 

 in the presence of a hexose and of thymine in the animal nucleic 

 acid, while the plant nucleic acid contains in their stead a pentose 

 and uracil. It is, however, significant that the only two exceptions 

 have so far been found in certain nucleic acids obtained from the 

 generative organs. The nucleic acid obtained from the eggs of a fish 

 (Gadus aigle/inus) contains uracil and a pentose but no thymine and 

 no hexose. 1 And while nucleic acid prepared from ripe spermatozoa 

 does not contain pentoses, these substances are stated to be present 

 in the nucleic acid of the testis of the bull, 2 which represents the 

 acid constituent of the nuclei of the sexual element in the various 

 stages of their development. Since the statement of the presence 

 of a pentose in the nucleic acid from the testis of the bull is 

 based only on the preparation of an osazone, further investigation 

 on this point and analytical data are necessary before it can be 

 accepted. 



The origin of the purine and pyrimidiue derivatives which form 

 part of the nucleic acid molecule is as yet obscure. In experiments 

 on the developing ovum it has been shown (see p. 282) that the 

 living cell has the power of synthesising these substances. 



This has been confirmed by experiments on Mammals by various 



1 Levene and Mandel, " Uber die Nukleinkorper des Eies des Schellfisches 

 (Gadus ceglefoius)," Zeitsch. f. physiol. Chem., vol. xlix., 1906. 



-' Steudel, "Cber die Kohlenhydratgruppe in der Nukleins ( aure," Zeitsch. f. 

 physiol. Chem., .vol. Ivi., 1908. 



