250 THE BIOLOGY OF BIRDS 



it sinks to zero after the cessation of laying ; and it finally 

 takes a high negative value in birds which have long ceased 

 to lay (Riddle and Harris, 191 8). 



But part of the yolk of the egg is of a protein nature, and 

 the size of the egg — affected by the amount of albumen as 

 well as by the amount of yolk — is probably influenced by 

 changes in the protein-metabolism of the hen bird. Giving 

 ring-doves (Streptopelia) small doses of quinine-sulphate 

 reduces the yolk size and the total size of the eggs, and it is 

 known that quinine reduces the destruction of the nitro- 

 genous components of the tissues, and would therefore 

 probably check the formation of albumen by the walls of the 

 oviduct and also the characteristic transformations of nitro- 

 genous compounds in the egg itself (Riddle and Anderson, 

 1918). 



The yolk consists of 



phosphatides 



neutral fats (chiefly lecithin) 



protein (chiefly vitellin) 



98-5 



the residue being mineral matter and water. 



Egg-Envelopes, — The term vitelline membrane is pro- 

 perly applied to the delicate non-cellular layer immediately 

 around the yolk-containing ovum. It is the cell- wall, and 

 is clearly seen in the young ovarian ovum. But matters do 

 not long remain so simple. As Lecaillon (1910) has shown, 

 there is outside the non-cellular vitelline membrane a median 

 layer of degenerating epithelium, and outside that again an 

 external layer of fibrillar connective tissue. The two layers 

 outside the true vitelline membrane are contributed by cells 

 of the ovarian foUicle. The whole envelope would be better 

 called the vitelline capsule. Its complexity is an adaptation 

 to the large amount of yolk which makes the ovum a much 

 bloated cell. The firmer capsule prevents bursting when 

 the egg is liberated from the ovary and begins to pass down 

 the oviduct. 



