SECTION I 



THE UNFERTILISED EGG AS A 

 PHYSICO-CHEMICAL SYSTEM 



I -I. Introduction 



In giving an account of the present state of our knowledge about the 

 chemical constitution of the egg-cell and the food-material which is 

 accumulated around it or inside it, I shall not follow a strictly logical 

 order of exposition, according to the phyla of systematic biology. 

 I have judged it best to begin with the egg of the hen, for not only 

 is it the most familiar and the best known of all eggs, but it is also 

 the one which has been most thoroughly investigated biochemically. 



It should be remembered that the two main morphological divisions 

 of the egg, (a) the egg-cell itself and {b) its coverings, appear in 

 protean modifications throughout the animal kingdom. The former 

 may be a simple cell with its ooplasm, nucleus, nucleolus, etc., as in 

 the echinoderms, and no covering at all save its cell-membrane, or 

 at the other extreme it may be swollen up with food-material or yolk 

 to the prodigious proportions of the avian egg-cell. The membrane 

 again may be a thin coat of investing cells such as the tunicate egg 

 possesses, or it may be the jelly of the amphibian egg, or, again, it 

 may be the complex arrangement of egg-white, chalazae, shell- 

 membranes, and shell, which is present in the bird's egg. All imagin- 

 able degrees of richness in yolk are present in the egg-cells of animals, 

 and upon this fact depend the various kinds of cleavage which they 

 show: alecithic eggs, on the one hand, such as those of most inverte- 

 brates, having a holoblastic form of development in which the whole 

 egg participates in cleavage; and yolk-rich eggs, on the other hand, 

 such as those of most vertebrates, having a meroblastic development, 

 only a localised part of the egg undergoing cleavage, the rest remaining 

 as a sac full of yolk until it is finally absorbed. 



1-2. General Characteristics of the Avian Egg 



After the historical introduction which has been given, it should be 

 unnecessary to remark on the general arrangement of the bird's egg. 

 We have with Harvey referred to it as an exposed, and, as it were, 

 detached uterus, and with Fabricius ab Aquapendente we have 



