SECT. 10] THE NITROGENOUS EXTRACTIVES 1159 



Without formulating any definite generalisation it may be said that 

 this association between nucleoprotein synthesis and terrestrial life is 

 probably no coincidence. In Section 9-15 it was shown that embryos 

 which develop in terrestrial, i.e. highly cleidoic, eggs excrete their 

 waste nitrogen mainly in the form of uric acid, and this fact was ad- 

 vanced as an explanation for the existence of uricotelic metabolism 

 in the birds, reptiles, and insects. Now if the uricotelic mechanism 

 is essentially due to the presence of the enzyme uricoligase, which will 

 synthesise the purine ring from lactic acid or some other three-carbon 

 chain and ammonium carbonate or urea, it is interesting to find that 

 wherever the uricotelic power exists, there also in the egg a notable 

 synthesis of purine bases for chromatin exists, and where ammonia 

 or urea are the end products of protein catabolism, there the egg is 

 provided with sufficient or nearly sufficient purine bases at the be- 

 ginning with which to construct its embryo. It is as if the synthesis 

 of the purine ring were relatively difficult, and just as in the mammals 

 there is a return to urea excretion after the uricotelic metabolism of 

 the reptiles, so in the transition from amphibia to sauropsida, supplies 

 of nuclein being no longer necessary in the egg there is a retention of 



this important material by the parent organism. It will be very 

 interesting to see if future work supports the provisional rule that only 

 uricotelic embryos synthesise nuclein. 



As for the means by which this is accomplished nothing is known. 



Russo regarded his results as 



supporting the theory of ar- 



ginine and histidine as pre- 

 cursors, but, as we have seen, 



Phmmer & Lowndes' results 



are in flat opposition to this 



view. Clemen ti has suggested 



that the protamines and his- 



tones, with their extremely 



high arginine-content, are in 



reality the intermediate pro- 

 ducts between the amino- 



acids of the yolk and the 



purine bases of the cellular nucleoproteins, but this is admittedly only 



a working hypothesis, and the subject is still one of the most obscure 



corners of the whole field. 



Days-* 5 



Fig- 354- 



