222 THE OHICK. 



inasmuch as it enables the embryo to develop rapidly and 

 securely, and frees it from the necessity of obtaining food from 

 without. However, it has also its disadvantages. Food-yolk is 

 itself inert, as already noticed in the opening chapter. It is 

 present in the egg as a number of granules of various shapes 

 and sizes, embedded in the living protoplasm of the egg ; and the 

 immediate effect of these inert, inactive, yolk-granules is, not to 

 aid, but to mechanically impede the processes of development ; 

 an effect which will necessarily be most marked in the early 

 stages, when the food-yolk is most abundant. Hence the early 

 stages of development of the chick, and especially the processes 

 of segmentation, occur more slowly than those of the frog, and 

 much more slowly than those of Amphioxus. 



Moreover, the amount of food-yolk in the hen's egg is so 

 great that serious distortion of the shape would occur, were the 

 whole mass contained within the body of the embryo. To avoid 

 this difficulty, the yolk, at a very early stage of development, 

 becomes constricted into two parts, embryonic and vitelline 

 respectively, which remain connected by a stalk. Of these (cf 

 Figs. 99 and 100), the embryonic portion, em, is formed from 

 the part of the egg comparatively free from food-yolk, and be- 

 comes converted directly into the embryo ; while the vitelline 

 portion or yolk-sac, YS, which contains the bulk of the food-yolk, 

 does not give rise directly to any part of the embryo, but forms 

 a store of nutriment at the expense of which the development 

 of the embryo is effected. 



At first, the embryonic portion is very much smaller than 

 the vitelline portion or yolk-sac (Fig. 98) ; but, inasmuch as 

 the embryo grows by absorption of the food-yolk, the yolk-sac 

 diminishes as the embryo increases in size (cf Figs. 99, 100, 101). 

 A time comes when the two are about equal in bulk, and in the 

 later days of incubation the yolk-sac is much smaller than the 

 embryo. By the twenty-first day of incubation the yolk-sac is 

 almost completely absorbed, and the chick pecks its way out of 

 the shell, and hatches. 



3. The Embryo. 



The hen's egg is fertilised before it is laid, indeed before the 

 egg-shell is formed, for no spermatozoon could possibly make 

 its way through the shell. At the time the egg is laid, not only 



