SECT. 9] 



PROTEIN METABOLISM 



1 129 



chick embryo during its development, a striking resemblance appears, 

 if it is assumed that the curve for the sheep and cow is the de- 

 scending limb of a peaked curve essentially similar to that occurring 

 in the chick embryo. If this view were adopted, we should have 

 to conclude that the protein utilisation peak was reached by the 

 sheep embryo some time before it attained the weight of 8 gm. 

 (i.e. about the 4th week out of 22), and by the cow embryo before 

 it attained the weight of 400 gm. (i.e. about the 12th week out of 32). 

 In this there is nothing improbable. On the other hand, as Lindsay 

 herself suggested, there is nothing to show that placental excretion 

 does not play a relatively larger part towards the end of development, 

 so that the fall in intensity of 

 protein combustion might be 

 purely an artifact. If this were 

 the case, a difference in urea 

 and uric acid content of the 

 foetal blood in early and late 

 pregnancy would be worth look- 

 ing for. But, whether this be so 

 or not, it is at any rate sugges- 

 tive that we have a curve for 

 the intensity of protein com- 

 bustion for the mammalian 

 embryo which looks as if it 

 might be in perfect correspond- 



ndsay) 



Cow 



1000 2000 3000 4000 5000 

 Weight of embryo ingms. 



Fig. 342. 



ence with that established for the oviparous avian embryo. It would 

 be interesting to make calculations about the total quantity of waste 

 nitrogen produced by the mammalian embryo during its intra- 

 uterine life, using the data for nitrogen content of amniotic and 

 allantoic liquids, and for urea content of maternal and foetal 

 blood, but such a calculation would be beset with so many diffi- 

 culties, and would involve so many assumptions, that it is not 

 worth beginning it in the present state of our knowledge. In view 

 of the arguments to be brought forward at the end of this chapter, 

 it might be predicted that the mammalian embryo would combust 

 much more protein than that of the chick, but it would be difficult 

 to know in what terms to express it, for the initial store could be 

 regarded as almost unlimited, and the total material combusted 

 would be extremely difficult to ascertain. Perhaps something could 



