872 



GENERAL METABOLISM 



[PT. Ill 



100 gm. of water there will be more solids present at the end of 

 development than at the beginning. The details of this concentration 

 process were worked out for the chick embryo by Murray in the 

 case of protein and fat, and by Needham in the case of total carbo- 

 hydrate. As Fig. 221 shows, there is the gradual rise that would be 

 expected a priori. It may be noticed that the only exception to this 

 rule occurs in the case of the total carbohydrate, for from the 4th to 

 the 8th day this value descends, showing the important part played 

 by carbohydrate in the embryo in the \ery early stages. The 

 protein uncorrected for that in the feathers has a peak on the 

 1 7th day, which is exactly what 

 might be expected, since the 

 feathers consist of practically dry 

 protein, and it is about the 1 7th 

 day that they form the highest 

 percentage of the body-weight. 

 The rise in the fat in relation to 

 the water recalls the "lipocytic 

 constant" and the other tissue 

 constants suggested by Mayer & 

 Schaeflferand Javillier & Allaire. 

 Their behaviour during the de- 

 velopment of the chick is very 

 interesting (see Section 12-5). 

 It will be important in the future 

 to investigate further the lipoid 

 and sterol content and concentration of the embryo, especially with 

 a view to unravelling their influence on surface phenomena in de- 

 velopment. The "Nachahmung" school of workers have already 

 shown how likely it is that surface factors are one of the most im- 

 portant foundations of morphogenesis; it remains to establish by 

 direct enquiry that this is really the case. 



The second important factor in the water metabolism of the de- 

 veloping avian egg is the loss of water through the shell to the 

 environment. At the opening of the Section on respiration a number 

 of citations were made which showed that the loss of weight which 

 occurs during the 3 weeks of incubation has been known for a long 

 time, ever since the eighteenth century. EstabHshed by so many 

 workers for the hen, Groebbels and Groebbels & Mobert have re- 



Fig. 221, 



