300 



EMBRYOLOGY. 



plates, the future vertebral arches 



Fig. 329. Ideal sections of figs. 327 

 and 328. A, tranverse section ; B, longi- 

 tudinal section; a, vitellary membrane; 

 6, serous layer of the blastoderm a, or ger- 

 minal membrane, depressed in the middle 

 by reason of the rounded elevations of 

 the dorsal laminae on either side ; e, chor- 

 da dorsalis ; c, mucous layer of the blasto- 

 derma ; d, vascular lamina, between b and 

 c, indicated by a finely-dotted line. 



Fig. 330. Vitellus of the natural size 

 after twenty-four hours of incubation, the 

 germinal membrane with the rudiments 

 of the embryo farther advanced than in 

 fig. 327. The references are the same in 

 this as in figure 327. 



(fig. 331, c, c, fig. 332, 

 A,/), which form at first 

 but three or four pairs ; 

 the cristse of the dorsal 

 laminae are observed to 

 approximate more and 

 more, in order to close 

 and complete the verte- 

 bral canal (fig. 332, A) 

 over the chorda dorsalis 

 (e). Anteriorly they se- 

 parate to a greater ex- 

 tent from each other to 

 form the head (fig. 331, 

 d), and also posteriorly 

 to form the future sa- 

 crum ; the enveloping 

 fold, the future involu- 

 crum capitis, is thrown 

 farther back (fig. 331, 

 e, e) ; the vascular and 

 mucous laminae of the 

 germinal membrane fol- 

 low this bending in (fig. 

 332, /), by which the 

 beginning of the intesti- 

 nal canal is produced, 

 which as yet is nothing 

 more than a depression 

 on the vitelline side of 

 the serous lamina of 

 the germinal membrane. 

 The embryo lies like 

 a flat-bottomed boat 

 turned over upon the 

 germinal membrane (fig. 

 332, B) ; the head is 

 already strongly indi- 

 cated (fig. 332, B, e). 

 [ 485. With the se- 



