308 THE DEVELOPMENT 



forms a nearly closed tube by the folding together of its edges. 

 In the head this tube is wide and deep, (at hd,) and there con- 

 stitutes the anterior end of the nervous cord, L e., the brain. 

 Consentaneously with the formation of the hollow nervous cord, 

 the juxtaposed edges of the halves of the vertebral layer become 

 elevated and form a sheath about it. Consequently, we find this 

 layer here, as seen in profile, at the same level with the upper 

 edge of the nervous cord, and extending below to the horizon of 

 the lower face of the chorda dorsalis (ch, ch 1 ). The extent of 

 this layer is rendered conspicuous, in this profile view, by the 

 presence of a few of the preliminary vertebrae, (v,) which have 

 developed from it by a transverse division of its whole thickness 

 into successive squares. The principal additional feature is ob- 

 servable in the inception of the intestinal layer (il). This is 

 formed by the separation of a stratum of cells from the upper 

 face of the subsidiary layer ; first at its central portion (si) along 

 the lower face of the body, and then extending centrifugally, not 

 only as far as the head, tail, and sides, but eventually beyond 

 these points, as we shall see presently. We may also see at this 

 period the manner in which the blood-system begins to form, 

 which is by a mere hollowing of channels in the upper surface 

 of the subsidiary layer. The first distinctly bounded cavity of 

 this system is that of the heart, (A,) which, almost from the be- 

 ginning of the formation of the blood-vessels, indicates its char- 

 acter by feeble pulsations. It is a simple, broad, and short tube 

 which lies lengthwise exactly in the lower mid-line of the body, 

 and a short distance behind the head. At the present period the 

 blood merely surges backward and forward in the channels, under 

 the influence of the contraction and expansion of the heart. 



I shall next lay before you the illustration of a phase of devel- 

 opment which is considerably in advance of the last one, but in 



Fig. 209. The same as figs. 204 to 208, in a more advanced state of develop- 

 ment than the last. 2 diain. A bird's-eye view of the embryo, vs, the periphery 

 of the yolk ; y, the yolk ; ?/' , the edge of the upper surface of y ; hd, the head ; tl, 

 the tail; n, the region of the nervous cord; ch, the anterior, and cA 1 , the poste- 

 rior end of the chorda dorsalis (cA 2 ) ; h, the heart ; vn, the median blood-vessel ; 



