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 jDrocesses of develop- 

 ment, the eiitire yolk, i.e. the egg proper, does not divid e, but 

 Rep mTgTita.tinn is vpstricted to a small circular pat ch (Fig. 97, BA), 

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

 yolk-granules, and in which development can readily take 

 place. Thi s patch, the sferminal 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 rapidlv increases in diameter, by growth all round 



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 pelluciila of the blastoderm. AK, area opaca. AV, area vasculosa. 

 SM, 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 (Fig.s. 

 98, 99, 100, 101). Owing, ^apparently, to its less sp ecific gravity, 

 the germinal disc, and conseque ntly the embryo, whic h is formed 

 from its cent ral par t, lies at thetop of thfi P.P'g. and nearest to 

 the body nf t^r hrnj hnwpv er much the egg b^ rnlled^ nvf^r 



The rPTitrflT jart of the blastode rm i s thin and translucent, 

 and is spoken of as t he area pellucida (Fig. 98, ad) ; the ma r- 

 gi nal portion i^_thicker_a ncL less t ransparent, and_ig_called the 

 a,rea opaca:. AK ; the inne nrim of the area opac a, bordering the 

 area pellucida, is the seat of an ab undan t_iQ£ matioii~of_ bloo d- 

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



