SECT, i] PHYSICO-CHEMICAL SYSTEM 235 



It is interesting in this connection that Riddle has traced the 

 occasional production of eggs with deficiency of white and shell but 

 not of yolk, to a lack of the thymus hormone which he has called 

 "Thymovidine". Feeding with desiccated thymus removed com- 

 pletely these effects. "The whole of the data", he said, "seem to 

 demonstrate the presence in the thymus of a substance having a highly 

 specific action on the oviduct of birds — and presumably on that of all 

 those vertebrate animals which secrete egg-envelopes." The syndrome 

 involved eggs with normal yolks but hardly any shell or albumen, 

 frequent reduction of normally paired ovulations to single ovulations, 

 diminished fertility, and restricted hatchability of the eggs. "Though 

 not necessary to the life of the individual", said Riddle, "thymo- 

 vidine would seem to be essential to the perpetuation of those verte- 

 brate species whose eggs are protected by egg-envelopes. Such 

 animals were the ancestors of mammals and thus mammals could 

 hardly have come into existence without the thymus." These con- 

 siderations are of much interest in view of other speculations on the 

 evolutionary aspect of chemical embryology, e.g. Section 6-6. They 

 also suggest that the mammalian thymus is now a vestigial organ. 



The air-space, the shell and the white of the normal egg need no 

 special remark at present, but the yolk is a more complicated structure. 

 Around a central core of "white" or "milky" yolk the yellow yolk is 

 secreted in the ovary of the hen in concentric layers, which form the 

 appearance of "haloes" in the finished egg, and which show up es- 

 pecially clearly when the hen is fed on Sudan III or some other non- 

 toxic dye which has a selective staining action on fat. The white yolk 

 in the centre is continued in a flask-like shape (the latebra) up to the 

 surface of the yolk underneath the germinal disc, and is then con- 

 tinued in a very thin layer all round the exterior of the yolk under- 

 neath the vitelline membrane. The white yolk is thus the first 

 nourishment of the embryo. It is not certain whether there are 

 also layers of white yolk between the concentric layers of yellow yolk, 

 for they have never been analysed chemically, and Balbiani main- 

 tains that they only differ from the yellow layers by having less yellow 

 pigment. The differences between the true white yolk and the yellow 

 yolk are, as will be seen later, far more profound. Balfour & 

 Foster, in their Elements of Embryology of 1877, described the yellow 

 yolk as consisting histologically of spheres of from 25 to loo/x in 

 diameter, filled with numerous minute highly refractive granules and 



