582 



GENERATION AND DEVELOPMENT. 



FIG. 216. 



(After Dalton.) Impregnated egg, 

 with commencement of formation of 

 ombryo ; showing the area germina- 

 tiva or embryonic spot, the area pel- 

 lucida, and the primitive groove or 

 trace. 



germinal membrane, the groove being wider at its anterior or 



cephalic extremity, and taper- 

 ing towards the opposite ex- 

 tremity. 



Coincidently with the forma- 

 tion of the primitive groove, two 

 oval masses of cells, the lamincK 

 dorsales, appear, one on each 

 side of the groove. At first 

 scarcely elevated above the 

 plane of the germinal mem- 

 brane, they soon rise into two 

 prominent masses, the upper 

 borders of which gradually tend 

 towards each other, turning in- 

 wards over the primitive groove. 

 The parts from opposite sides 

 then unite, and convert the 

 primitive groove into a <tube, 

 large and rounded in front, nar- 

 row and lancet-shaped behind, which is the central canal of 

 the cerebro-spinal axis, and contains the rudimental spinal 

 cord and brain, which are developed in its interior (Fig. 217). 

 Immediately beneath, and in a line parallel with the primi- 

 tive groove, may be seen, about the same time, a narrow linear 

 mass of cells, the chorda dorsalis, which forms the basis around 

 which the bodies of the vertebrae are developed. The devel- 

 opment of this column is early indicated by the appearance 

 of a few square, at first indistinct, plates, the rudiments of 

 vertebrae (Fig. 217, D), which begin to appear at about the 

 middle of each dorsal lamina. 



While the dorsal laminae are closing over the primitive 

 groove, thickened prolongations of the same serous layer are 

 given off from the lower margin of each of them, and are 

 named lamince viscerales sen ventrales. These visceral laminae 

 by degrees bend downwards and inwards, and at length, in- 

 closing a part of the yelk, unite and form the anterior walls 

 of the trunk inclosing the abdominal cavity below, as the 

 dorsal plates inclose the cerebro-spinal canal above. 



Umbilical Vesicle. 



The ventral laminae, as they extend downwards and inwards, 

 at first proceed on the same plane with the inner layer of 

 the germinal membrane, which immediately lines them. Soon, 

 however, they show a tendency to turn inwards, so as to con- 



