644 EMBRYOLOGY 



in the latter of a considerable quantity of yolk or food material 

 stored to provide for the nourishment of the embryo during the 

 early stages of development. This yolk, lying on one side of the 

 egg, hampers the relatively scanty protoplasm there, which 

 therefore divides more slowly, and, as we have seen, forms fewer 

 and therefore larger cells than the other side. In the dogfish 

 and in birds there is no food-procuring, larval stage of 



development, but the embryo is 

 nourished within the egg until it 

 has substantially the features of 

 the adult. Accordingly the yolk 

 is still more plentiful, with the result 

 that the portion of the egg in which it 

 is stored never divides at all, but 

 remains as an inert mass until it is 

 surrounded by the growth of the small 

 protoplasmic region or germinal disc, 

 which, containing the nucleus, lies 

 originally at one pole (Fig. 500), and 

 segments to form the cells of the 

 embryo. The cleavage of the ovum 

 of the lancelet is complete or holo- 

 blastic and almost equal ; that of the 

 ovum of the frog is holoblastic and 

 unequal ; that of the dogfish and birds is incomplete or 

 meroblastic. 



Fig. 500. — A diagrammatic 

 section of the egg of a 

 bird. — From Thomson. 



a.c,, Air chamber ; ch., twisted cords in 

 the white known as ' chalazae ' ; 

 g.v., ' germinal disc,' a small patch 

 of protoplasm, comparatively free 

 from yolk, in which lies the 

 ' germinal vesicle ' or nucleus ; 

 y., yolk, in alternate layers of 

 yellow and white substance. The 

 yolk is surrounded by the ' white 

 of egg.' Note the two membranes 

 underlying the shell and separating 

 to enclose the air chamber. 



DEVELOPMENT OF THE CHICK 



The egg of a bird — for instance, that of the common fowl 

 (Fig. 500) — consists of an immense ovum, the so-called ' yolk ', 

 with certain coverings. The ovum owes its size to being swollen, 

 as we have just seen, by a great quantity of nutritive material, 

 the yolk proper, which pushes the bulk of the protoplasm to 

 one side as the germinal disc. It is covered, first by its closely- 

 fitting vitelline membrane ; then by the ' white ', which is a 

 solution of proteins and salts ; then by a double membrane, 

 whose two layers, when the egg has been laid for some time, 

 part at the broad end and so enclose an air-space ; and finally 

 by a porous, chalky shell. The white provides an additional 

 store of nutriment, but its principal function is to serve as a store 



