SECT. I] PHYSICO-CHEMICAL SYSTEM 285 



of nidifugous birds. This must imply that the greater requirement 

 for nutrient material in the latter case has, as it were, packed the 

 fat tighter into the yolk. Exactly the same relationship is brought 

 out from the figures of Spohn & Riddle, who compared the pigeon 

 which hatches out as a squab with the hen which hatches out as a 

 fully-feathered chick. Spohn & Riddle's analyses are the only com- 

 plete ones we have for a nidicolous egg, and bear clearly the same 

 relationship, for there is less protein and less fat, relatively, in the 

 pigeon's egg than in the hen's. The ash content and the amount of 

 non-nitrogenous extractive substances seem, however, to be slightly 

 higher in the latter case. Langworthy's figures were all obtained 

 from the eggs of nidifugous birds, and they show a great similarity 

 among themselves. More delicate consideration, of course, reveals 

 differences according to breed in the hen's egg, e.g. the figures of 

 Pennington and his collaborators, but these are of a comparatively 

 minor order. 



The most interesting analyses are those of Spohn & Riddle. They 

 compared the egg of the jungle-fowl, which is supposed to have been 

 the evolutionary ancestor of the domestic hen, with averaged figures 

 for hen's eggs of various breeds, and, as is evident, there was a very 

 close agreement. They also analysed the white yolk as distinct from 

 the yellow yolk of the hen's tgg. When the yolk begins to be formed 

 in the ovary of the hen, it is white and not yellow, and not until 

 the critical point in its maturation is reached, when its growth-rate 

 completely changes, does it begin to store lipochrome pigment. This 

 change in growth-rate, which has been observed by other workers 

 as well as Riddle (e.g. Walton), will be dealt with in more detail 

 in the appendix on maturation. Von Hemsbach, in a paper on the 

 milky or white yolk of the birds, in 1851, suggested that the corpus 

 luteum of mammals corresponded to the yellow yolk of birds, and 

 that the mammalian ovum having been shed out of the ovary into 

 the Fallopian tube and uterus, the fats and lipochrome pigment 

 were laid down in the Graafian follicle instead of around the white 

 *'ovum". Von Hemsbach also supported the view already mentioned 

 that the shells of avian and amphibian eggs corresponded to the 

 decidua of mammals. He laid stress on the work of Zwicky and 

 Gobel, who had investigated the pigments of yolk and corpus luteum, 

 and had thought them to be identical. This subject will be referred 

 to again under the head of pigments. 



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