920 GENERAL METABOLISM [pt. iii 



in, and absorbing, the yolk, complicates the process further. Then, 

 as the mass of yolk becomes gradually more and more enormous 

 relative to the embryo, the walls of a special yolk-sac take on 

 absorptive functions, and in birds, for example, become of great 

 importance. The monotremes show the abandonment of the inter- 

 mediate methods^. 



We may now return to the rate of absorption of the yolk in the 

 hen's egg. 



An absorption-coefficient could not be calculated from the constitu- 

 tion figures only, for the rate of combustion of certain substances and 

 the rate of transformation of others might, and in fact actually does, 

 vary considerably. The only way to find out the relative absorption 

 intensity of the various substances of the food-supply by the embryo 

 is to make a large number of analyses and special calculations. So 

 far the chick is the only embryo for which this has been possible. 

 Murray's work provided the greater part of a sound foundation for 

 these computations, and I made them in 1926 and 1927. 



We may take first the absorption intensity of protein throughout 

 development. To calculate it, it was necessary first of all to know 

 the amount of true protein inside and outside the embryo on each 

 day of incubation, and this was obtained by making various cor- 

 rections ; thus from the total nitrogen of the embryo were subtracted 

 {a) the lipoid nitrogen, calculated from the work of Plimmer & Scott, 

 and of Masai & Fukutomi, (b) the purine nitrogen, calculated from 

 the work of LeBreton & Schaeffer, and {c) the water-soluble non- 

 protein nitrogen, taken from the work of Needham. For the total 

 protein nitrogen outside the embryo, the figures of Sakuragi were 

 used, after being corrected for the use of trichloracetic acid instead 

 of acetic acid and heat. The way these data were used is shown in 

 Table iii. Columns 2, 3, and 4 concern the embryo and need no 

 special remark. 



^ A most interesting sidelight on the nature of yolk-absorption is given by the work 

 of Newman on hybrids of the minnows Fundulus majalis and Fundulus heteroclitus. The 

 former fish lays eggs 2-7 mm. in diameter, the latter 2 mm. An F. majalis ? x F. hetero- 

 clitus ^ cross gives after about a month a hybrid embryo resembling the paternal 

 species, and too small for its yolk, i.e. unable to assimilate it. Although in the presence 

 of excess food, its absorption-mechanisms seem incapable of dealing with it and the 

 hybrid, sinking to the bottom, dies before it can hatch. The opposite hybrid (where the 

 egg is small) hatches small, unable to reach the large size of its paternal origin, but is 

 quite viable (cf. the genetic differences in avian yolk-absorption-rate mentioned on 

 p. 940). 



