SECT, i] PHYSICO-CHEMICAL SYSTEM 247 



in question. Nor are such comparatively slowly changing factors 

 the only ones which bring about differences between individual 

 eggs ; the time the egg takes to pass down the oviduct, for instance, 

 will materially affect the amount of albumen it contains, and 

 such variable quantities as the blood-sugar level (Riddle) and the 

 level of cholesterinaemia in the parent animal will exercise their 

 effects upon the resulting egg. Again, the length of time elapsing 

 between the laying of the egg and the beginning of incubation will 

 have a marked efTect, for a certain amount of water will evaporate 

 from the egg-contents through the shell, and just how much does so 

 will depend on the humidity of the surrounding atmosphere. The 

 process of water-absorption by the yolk (Greenlee) from the white 

 will also be affected by these conditions, so that the embryo at 

 the initiation of its incubatory development may find a remarkably 

 inconstant set of circumstances in its immediate environment. More- 

 over, a certain amount of development always takes place in the egg 

 after fertilisation as it passes down the oviduct, so that the embryo 

 has already gastrulated by the time that the egg is laid by the hen. 

 It was the ignorance of these facts which led Malpighi, as we have 

 already seen, to his erroneous conclusions, for if he had known of 

 the phenomenon of "body-heating", as it is called by the poultry- 

 farmer, he would not have put forward the preformation-theory, and 

 the eighteenth century would have been spared the trouble of getting 

 rid of that embryological phlogiston. Thus no two eggs are ever 

 exactly the same age, and as there is reason to believe that enzymic 

 action begins in the yolk, if not in the white, very shortly after 

 fertilisation, this fact makes it additionally difficult to get precise figures 

 for the constitution of the unincubated egg. Then the position of the 

 egg in the clutch (whether first or second) in pigeons may, according 

 to Riddle, make a difference of 9-15 per cent, in yolk weight. It 

 may be concluded that nothing short of the greatest caution must 

 be employed in the material which is used for chemico-embryological 

 researches on the hen's egg. The individual hens should be marked, 

 and the eggs produced by them should be noted, their food should 

 be constant in composition and the breed used should be not only 

 single in any one series of experiments, but also, if possible, genetically 

 pure. It is very greatly to be wished that standard hens could be 

 obtained, such as the standard rats necessary for feeding experiments, 

 and much further work, with a proper statistical backing, is needed 



