304 



EMBRYOLOGY. 



ribs likewise; these lateral prolongations of the serous Ja- 



mina are called 

 the lamina ven- 

 tral es, ventral 

 laminae. As the 

 dorsal laminae 

 arise more per- 

 pendicularly in 

 plaits, and con- 

 verge to close the 

 spi-nal canal, so 

 the ventral la- 

 minae spread 

 more in breadth, 

 bend in inferior- 

 ly, and converge 

 Fig. 335. Ideal sections of the embryo of fig ^o form the la- 

 330 ; letters of reference as in fig. 329. A, over ^^ par i etes OI 

 the chorda dorsalis, e, is seen g, the canal for the * , , 



spinal cord, formed by the union of the cristae of abdomen, 



the dorsal laminae. B, longitudinal section. The and finally to 

 heart, d 2 , is evolved as a thickening of the lamina close this cavity, 

 vasculosa. The vascular and 



mucous layers follow the turnings and general course of the 

 serous layer, and decline anteriorly under the head of the 

 embryo, by which the fovea cardiaca, the anterior depres- 

 sion which marks the commencement of the intestinal canal, 

 becomes deeper (figs. 332, B, /, and 335, B). From this 

 sinus the vascular and mucous layers turn more posteriorly, 

 and immediately again proceed forwards, to be continued 

 in the plane of the germinal membrane (fig. 335, B, where 

 the heart, d 2 , is indicated). This part of the germinal mem- 

 brane, then, covers the head of the embryo when it is viewed 

 from below, and on this account is called the involucrum capi- 

 tis the cranial envelope or cap among writers on develop- 

 ment ; it is not any independent formation. 



Whilst these changes in the form of the serous layer are 

 going on, others are proceeding, pari passu, in the vascular 

 lamina, in the following order, from the end of the first day 



blastoderma, from which the involucrum capitis is formed, shining through ; 

 g, posterior fold of the blastoderma, still very narrow, from which is 

 formed the involucrum caudse ; /, chorda dorsalis 



