912 GENERAL METABOLISM [pt. iii 



directly to the question of the absorption curves for the various 

 substances, i.e. the determination of the relative intensities with 

 which various substances are absorbed from the yolk or the non- 

 embryonic parts of the egg. These problems, again, cannot be dealt 

 with without discussing the efficiency with which the raw materials 

 are transformed into the finished embryo at different stages. All these 

 subjects can best be treated in relation to the egg of the hen, for it 

 is there that they have been most carefully worked out; in fact, it is 

 only in the case of the chick, where the embryo can be separated from 

 the yolk from a comparatively early stage, that these problems have 

 been even partially solved. 



First, as regards the percentage constitution of the embryo. 

 Table io8 summarises the accurate data which we have on this 

 question. It may be doubted whether in a computation such as this, 

 in which it is desired to compare active strengths of constituents in 

 the embryo, it is advisable to include substances packed away in 

 an immobile form. Glycogen would not come under this category, 

 nor would the adipose tissue of fat depots, but the keratin in the 

 feathers does seem somewhat remote from the balance of reactions in 

 the body. Murray, whose suggestion this was, estimated the amount 

 of feather substance present each day after the 12th, and his figures 

 are represented in Col. 7. He only gave these values, however, as 

 percentages of the body weight, so the actual weights are seen in 

 Col. 8, the corresponding amounts of keratin (assuming the feathers 

 to be 90 per cent, keratin) in Col. 9, and the true protein, excluding 

 the feather protein, in Col. 10. Col. 11 reproduces Col. 6 corrected 

 for the feathers, and finally Col. 12 shows the fat and Col. 3 the 

 carbohydrate in grams per cent, of dry weight. 



The graphic representation of this table is to be found in Fig. 244. 

 The curves make up an interesting assemblage, for all three have 

 peaks, and it is important to observe where they come. The carbo- 

 hydrate one occurs on or before the 5th day, the protein one on 

 the nth day, and the fat one on the 20th day. This is reminiscent 

 of the order in which, as will be seen (p. 993 j, the intensities 

 of combustion run; carbohydrate about the 5th day, protein at 

 8*5 days, and fat towards the very end of development. The way 

 in which fat overtakes protein at the end of development was first 

 noticed by Murray, and Riddle observed in the pigeons' egg a pre- 

 ferential absorption of fat from the yolk at that time. 



