940 



GENERAL METABOLISM 



[PT, III 



Spare "" 



Yolk Chick 

 • O Faverolle] 

 B D Gudan MLjin 



that after hatching it is entirely used for purposes of combustion, we 

 cannot avoid the conclusion that the catabolism of fatty acids which 

 was so prominent a feature of the pre-natal period, gives place to 

 combustions in which protein plays a greater part, as soon as hatching 

 is completed. Nor is it unwarranted to see in this arrangement a 

 state of affairs quite in accord with the fact that disposal of nitro- 

 genous waste-products is not easy 

 before hatching and is easy after- 

 wards. For further discussion on 

 this subject see Section 9-15. 



Referring again to Fig. 255 it 

 will be noticed that if the chicks 

 were given food a day or two after 

 hatching, there was hardly any 

 diminution in the dry weight of 

 the spare yolk, as is indicated by 

 the dotted line. No doubt it takes 

 a long time to disappear com- 

 pletely if the circumstances of 

 post-natal life are favourable to 

 survival. "Extracting the spare 

 yolk from the stomach", said 

 Iljin, "we saw that it is included 

 in a special tunicle like a sack. 

 This sack opens into the thin gut 

 by means of a special connection, 

 the inside surface of which is corrugated, like the bile-duct of man. 

 The wall of the gut makes a wrinkle above the opening which 

 covers it like a valve, so that the contents of the gut cannot enter the 

 duct during the peristaltic movements." 



That the spare yolk forms a considerable part by weight^ of the 

 newly hatched chick is clear, in any case, from Iljin's data, which 

 show from 13-6 to 27-8 per cent, of the wet weight and from 23-1 to 

 52-2 per cent, of the dry weight. 



Schillings 

 Bleecker 



Hatclning 



Fig- 255- 



^ The exact degree to which yolk is assimilated prior to hatching has been investigated 

 by Jull & Heywang. There is no sex difference in the amount of spare yolk, but con- 

 siderable variation according to the hen laying the eggs. The average amount of spare 

 yolk is 40-78 % of the original yolk, but it may vary from 32*14 to 46-88 %, and as the 

 eggs from individual hens are fairly uniform in this respect, the phenomenon appears to 

 be genetic in origin (cf. the teleostean hybrids described on p. 920). 



