224 THE CHICK. 



oviduct, a period averaging, as we have seen, about eighteen 

 hours. 



Owing to the enormous amount of food-yolk, and the me- 

 chanical hindrance which this offers to the processes of develop- 

 ment, the entire yolk, i.e. the egg proper, does not divide, but 

 segmentation is restricted to a small circular patch (Fig. 97, BA), 

 on the surface of the yolk, which is comparatively free from 

 yolk-granules, and in which development can readily take 

 place. This patch, the germinal disc, segments to form the 

 blastoderm, a membrane composed of cells (Fig. 106), which lies 

 like an inverted watch-glass on the surface of the yolk. The 

 blastoderm rapidly increases in diameter, by growth all round 



AD 



FIG. 99. The yolk of a Hen's Egg at the end of the third day of incubation. 

 The structure of the embryo at this stage is shown on a larger scale in 

 Figs. 113 and 114. x |. 



AD, area pellucitla of the blastoderm. AK, area opaca. AV. urea vasculowi. 

 EM, embryo. SM. vitelline membrane. 



its margin, and spreads so as to cover more and more of the 

 surface of the yolk, which it ultimately incloses completely (Figs. 

 98, 99, 100, 101). Owing, apparently, to its less specific gravity, 

 the germinal disc, and consequently the embryo, which is formed 

 from its central part, lies at the top of the egg, and nearest to 

 the body of the hen, however much the egg be rolled over. 



The central part of the blastoderm is thin and translucent, 

 and is spoken of as the area pellucida (Fig. 98, AD) ; the mar- 

 ginal portion is thicker and less transparent, and is called the 

 area opaca, AK ; the inner rim of the area opaca, bordering the 

 area pellucida, is the seat of an abundant formation of blood- 

 vessels, and is called in consequence the area vasculosa. AV. 



