SECT 6] OF THE EMBRYO 939 



mgm. which would correspond at the outside to 0-054 mgm. carbon 

 dioxide or 0-0146 mgm. carbon. And the hatched larva weighing 

 2 mgm. has about 0-6 mgm. of protein in it, or 0-3 mg. carbon, so 

 that only about 5 per cent, of the carbon absorbed was burned. 

 The P.E.C. was therefore about 95 per cent, or 0-95 but of course 

 hatching in amphibia is not the end of development, and before all 

 the remaining yolk is used up the efficiency will have fallen to the 

 usual 60 per cent, or 0-6. 



6-1 1. Metabolism of the Avian Spare Yolk 



This will be a convenient place in which to give an account of 

 the yolk which is still available for the chick at the time of hatching. 

 In the succeeding sections of the book, the metabolism of the yolk 

 and white during the pre-natal stages within the egg will be fully 

 unravelled, in so far as this is possible in the present state of our 

 knowledge, and the data will be found in their appropriate places 

 according to the section-headings. But the "spare yolk", as it is 

 called, has been investigated so little, and its significance so seldom 

 discussed, that it may as well be dealt with here. The most complete 

 study of it is that of Iljin, though it was first attacked by Virchow. 

 Iljin allowed newly hatched chicks to go without food for a number 

 of days and from time to time weighed the bodies and the 

 unutilised spare yolk. His data, which are plotted in Fig. 255 

 show that although the dry weight of the yolk diminishes enormously 

 during the first few days after hatching, the dry weight of the 

 chick remains practically constant. This yields an important 

 result, namely, that the chick has absorbed all that it required for 

 architectural purposes from the yolk before it hatches, and that after 

 that moment, the "spare yolk" plays an entirely nutritive part, 

 functioning mainly if not entirely as combustible material. "The 

 chicken Hves on its yolk", said Iljin, "and does not destroy the 

 organised parts of its body, which have only just been formed." Now 

 as will appear in later sections, and largely owing to the work of 

 Riddle, we know that during the last week of incubation, the chick 

 absorbs material from the yolk in varying intensity, lipoids being 

 assimilated more rapidly than fats, and neutral fat more rapidly 

 than proteins. It looks, therefore, as if the yolk at the time of hatching 

 is much more preponderantly composed of protein than it was at 

 the beginning of incubation, and as we know from Iljin's experiments 



