2i6 THE CHICK. 



pellucida. Be yond the edge of the blastoder m (Fig. 97) the 

 y olk shows o ne or more broa d concentric bands, alternat ely 

 d arker and lighter m appearanc e. 



Sections of the blastoderm at this stage (Fig. 106) show- 

 that it consists of two distinct layers of cells. A"^ Thn nppp.r 

 l ayer, or epiblast. e. is a continuous membrane, formed of small, 

 short columnar cells, varying very little in size, and packed 

 closely together side by side. 



(ii) T he lowe T- lay'^r onnsi.cjf . c! nf np.Hs which are more loo sely 

 arranged , and which vary a good deal in shape and size in 

 different parts. In the area pellucida, o r m iddle portion o f the 

 bl astoderm , they form a thin layer of somewhat flattened cells, 

 H, only one, or at most two cells in thickness. At the margin of 

 the blastoderm, or area opaca, the cells become more numerous 

 and more spherical in shape, forming a thickened rim which rests 

 on the underlying yolk, and in which the larg e formative cells, 

 ZFj_ai"e found, es pecially near the posterior margin , in the 

 yolk, on which the edge ol the blastoderm rests, nuclei (Pig. 

 106, nQ are present, round which cells are formed at a later 

 stage, and added on to the margin of the blastoderm. 



Beneath the area pellucida, and separating it from the bed 

 of yolk, Y, is the subgerminal cavity, BV ; a well-marked space, 

 filled with fluid. 



h^ Ttifi g'rn^.Ti nf t he blastoderm. Round the margin of the 

 bla stoderm the epiblast and the lower-lay er cells are at first c on- 

 ti nuous with each othe r, bu t shortly before the laying of the eg g 

 thisc ontinuitv is los t, exce pt at the posterior bord eri^ where, as 

 shown on the left-hand side of Fig. 106, the two layers are still 

 continuous with each other at the time the egg is laid. 



After incubation has commenced, the blastoderm spreads 

 rapidly, retaining its circular shape. By the end of the first 

 day of incubation it is about the size of a sixpence ; and by the 

 end of the second day it has extended nearly half way round 

 the egg; after this it proceeds more slowly, the complete 

 inclosure of the yolk not being effected until about the seven- 

 teenth day. 



In this spreading of the blastoderm (cf. Figs. 98 and 99) 

 th e perip heral part, or jirea opaca. gr ows much Tnm-e ra.pidly th an 

 th e^ central a r ea pellucida ; the area opaca re ta ins its circu lar 

 outline, but tl Te area pellucida (F igs. 98 and 99, ad) 



