236 THE CHICK. 



pellucida. Beyond the edge of the blastoderm (Fig. 97) the 

 yolk shows one or more broad concentric bands, alternately 

 darker and lighter in appearance. 



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

 that it consists of two distinct layers of cells, (i) The upper 

 layer, 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) The lower layer consists of cells which are more loosely 

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

 different parts. In the area pellucida, or middle portion of the 

 blastoderm, 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 large formative cells, 

 zf, are found, especially near the posterior margin. In the 

 yolk, on which the edge of the blastoderm rests, nuclei (Fig. 

 106, n') 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. 



b. The growth, of the blastoderm. Round the margin of the 

 blastoderm the epiblast and the lower-layer cells are at first con- 

 tinuous with each other, but shortly before the laying of the egg 

 this continuity is lost, except at the posterior border, 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) 

 the peripheral part, or area opaca, grows much more rapidly than 

 the central area pellucida ; the area opaca retains its circular 

 outline, but the area pellucida (Figs. 98 and 99, ad) very early 



