9i6 



GENERAL METABOLISM 



[PT. Ill 



not show such a fall in the case of the mammalian embryo. Possibly 

 this is due to the fact that we have no analyses of mammalian embryos 

 in the earlier stages, and the earlier, sharper part of the inorganic 

 curve may thus have been missed. Inspection of Col. 13 of Table 108 

 shows that this decline in ash-content percentage dry weight is in fact 

 more rapid at the beginning than at the end of development. Some 

 fragmentary data for the Jersey cow embryo contained in the paper of 

 Moulton, Trowbridge & Haigh, do not seem to show any change 

 between the 1 85th day of gestation and birth : 



Percentage 



'arying balance of the chemical 



Popov has also made analyses of cow embryos, but I have not been 

 able to gain access to his data. 



Another way of looking at the 

 constituents of the developing em- 

 bryo is to enquire whether the 

 ratios between any two of them 

 change with age. This method 

 was first used by Murray. "Before 

 undertaking my experiments", he 

 said, "I was impressed by what 

 seemed to be a natural scale or 

 gradient, as judged by various 

 criteria, of the chief groups of 

 substances under consideration, 

 namely, salt, carbohydrate, pro- 

 tein, and fat. A tentative predic- 

 tion was considered, that the 

 following ratios would be found to 

 decrease with age during ontogeny 

 water/ solid, inorganic/ organic, 

 carbohydrate/protein and pro- 

 tein/fat." The analyses which Murray and Needham subsequently 

 made confirmed this prediction in every particular. Murray himself 

 attempted to get the carbohydrate/protein ratio by estimating the 

 amount of glycogen in the chick embryo at different stages, but 



400 



300 



..j^ Carbo hydra be 



Protein 

 + Probe! n 

 • Fat 



Days-*5 



10 

 Fig. 249. 



