OF VERTEBRATA. 305 



shall show presently step by step, every organ takes its origin 

 from it by a separation of a portion of the thickness of its upper 

 side. I might say, therefore, in the strictest sense, that the ger- 

 minal layer (gl, gl 1 ) is the first to split off from the subsidiary 

 layer. We have, then, in this phase, the following features, 

 namely, the yolk sac (vs) partially occupied by liquified albumen 

 (al l ) which has infiltrated through its tissue ; the whole mass 

 of the yolk (the egg proper) removed from the centre of the 

 white (a/) and buoyed up near its upper side, taking the 

 place of the absorbed albumen ; the germinal disc (gl) and its 

 peripheric extension, the germinal layer (gl l ) ; and finally, the 

 subsidiary layer (si, si 1 ) from which the germinal layer (gl, gl 1 ) 

 split off. As yet there is no indication of right and left, nor of 

 head and tail, in the germinal disc. These two features develop 

 almost simultaneously. The latter, however, is the first to ap- 

 pear, and may be recognized as a __a_ _*LsL 

 thickening (fig. 205, hd) of one 

 edffe of the disc, and a sinking of 

 the germinal layer at that point 

 and also at the opposite edge, (am,) 

 so that two shallow furrows are 

 formed. The former becomes ev- 

 ident before the latter has com- 

 pletely developed itself, and may 

 be recognized as a faint, straight, 



shallow furrow which passes through the middle and at the 

 upper surface of the germinal disc (gl). This is called the prim- 

 itive stripe, and corresponds to the axis of the body. 



In the next phase (figs. 206, 207) the characteristic feature of 

 Vertebrata makes its first appearance ; the primary element - 

 the so-called chorda dorsalis of the vertebral column is formed. 



Fig. 205. The same as fig. 204, with the shell and white taken off, but further 

 developed. 2 diam. y, the yolk ; r/ 1 , surface of y ; al l , clear, albuminous space ; 

 gl, the germinal disc ; gl 1 , gP, the extension of gl beyond the body ; lid, the 

 head; tl, the tail; si, si 1 , sP, the subsidiary layer; am, the furrow around gl ; 



1 , the peripheric fold of am. Original, 



20 



