220 THE CHICK. 



This theory of Preformation was vigorously combated by 

 Caspar Friedrich Wolff, who in 1759, when only twenty-six 

 years old, published as a thesis for the doctor's degree his theory 

 of Epigenesis, which offered an entirely new explanation of the 

 mode of development of the chick and other animals. Wolff 

 showed conclusively that in the hen's egg, as laid, there is no 

 trace whatever of the embryo, or of any of its parts or organs ; 

 and that the formation of the embryo does not commence until 

 after the egg is laid and incubation has begun. He noted 

 further, and described accurately, the manner in which the 

 embryo is formed by folding of the germinal layers or 

 membranes. 



Wolff was too far ahead of his age, and his conclusions, 

 though perfectly sound, did not obtain acceptance until towards 

 the middle of the present century, when their correctness was 

 demonstrated, not merely for the chick, but for many other 

 groups of animals as well, by von Baer, Remak, Bischoff, 

 Kolliker, and others. 



Although the chick has thus played a more important part 

 in the history of embryology than any other animal, it must be 

 borne in mind that birds are one of the most highly specialised 

 groups of animals, and that their development is, more particu- 

 larly in the early stages, very greatly modified. It is practical 

 convenience alone that justifies the great attention they have 

 received. 



2. The Egg. 



The hen's egg is of large size, and ovoid in shape. It con- 

 sists (Fig. 97) of a calcareous shell, lined by a fibrous shell 

 membrane ; and inclosing a quantity of a viscid albuminous 

 fluid, the ' white of the egg,' WA, in the centre of which lies 

 the yolk,' Y, a spherical mass of a yellow colour, rather more 

 than an inch in diameter, and inclosed in an elastic vitelline 

 membrane, to which the preservation of its shape is due. 



Of these parts, the yolk is the egg proper ; it corresponds 

 to the egg of Amphioxus, or of the frog, and from it the embryo 

 is developed directly. 



The white of the egg corresponds to the investment of the 

 egg of Amphioxus, which swells up so greatly on reaching the 

 water, or to the jelly of the frog's spawn. The egg-shell and 



