OF VERTEBRATA. 307 



furrowed deeper into the head and thinning out, i. e., growing 

 shallower toward the mid-length of the body ; the chorda dor- 

 Sfilis (ch, ch 1 ) lying between the right and left halves of the body, 

 and just beneath the primitive stripe (ps) ; the vertebral layer, 

 (r,) upon which the back-bone is eventually developed, com- 

 pletely split off from the upper surface of the subsidiary layer 

 (si) and lying in equal parts right and left of the chorda dorsalis 

 (ch) ; the subsidiary layer (si) lying close to the under face of 

 the disc, and slightly depressed by the downward curvature of 

 the head and tail, but separated from the rising fold of the ger- 

 minal layer, and extending directly (at sl } , si' 2 ) to the periphery 

 of the yolk. 



In the next phase (fig. 208) it is clear that the embryo has ad- 

 vanced considerably both in regard to the development of those 

 organs already initiated, and by 

 the institution of new features. 

 The body is nearly enveloped by 

 the uprising and contracting edge 

 (am 1 , am">) of the amniotic sac. 

 The head (hd) and tail (tl) are 

 deeply sunken toward the centre 

 of the yolk mass, (y,} and so 

 strongly incurved toward each 

 other as to form a cavity between Fig. 208. 



them, which opens on the lower side into the great area occupied 

 by the albumen, (al 1 ,) and on the upper is bounded by the inferior 

 surface of that portion of the subsidiary layer (si) which lies 

 between the two ends of the body. The furrow of the primitive 

 stripe extends more than two thirds of the length of the body, and 



Fig. 208. The same as figs. 204 to 207, but further advanced. A profile 

 view. 2 diam. y, the yolk; y l , the surface of y; a/ 1 , the clear, albuminous 

 space; gl. the original germinal disc, now, more properly speaking, the body; 

 hd, the head; tl, the tail; ch, the anterior, and ch 1 , the posterior end of the 

 chorda dorsalis ; v, the primitive vertebra ; aw 1 , aw 3 , the edges of the fold of the 

 amniotic sac; am 1 , am-, the space beneath aw 1 , am 3 ; i/, the intestinal layer; h, 

 the heart, a channel in the intestinal layer (it) ; *l, the subsidiary layer; si*, sP, 

 that part of the subsidiary layer in which the vascular area is formed. Original. 



