FORMATION OF DECIDUA, AND VILLI OF CHORION. 



457 



which the tufts of the chorion are received; whilst its follicles are 

 enlarged into glandulae of sufficient size, to allow these villi (in some 

 Mammals at least) to extend themselves into their interior. In its 

 earliest grade of development, as already remarked, the chorion and 

 its villi contain no vessels; and the fluid drawn in by the tufts is 

 communicated to the embryo, by the absorbing powers of its germinal 

 membrane. But when the tufts are penetrated by blood-vessels, and 

 their communication with the embryo becomes much more direct, the 

 means by which they communicate with the parent are found to be 

 essentially the same ; namely, a double layer of cells, one layer 

 belonging to the foetal tuft, the other to the vascular maternal surface. 

 (See 819.) 



812. We now return to the Embryo itself; the general history of 

 whose development has been already traced, up to the period at which 

 the cluster of cells in the Area Germinativa is about to give origin to 

 the permanent structures of the foetus. The parts first formed in the 

 embryo of Vertebrated animals, are such as most characteristically 

 distinguish them from all others ; namely, the Vertebral Column, and 

 Spinal Cord. The first indication of these consists in the formation 

 of what is termed the primitive trace, which is a shallow groove, lying 

 between two oval ridges (Fig. 141, v\ known as the lamince dorsales. 

 The form of these is changed with that of the area pellucida ; at first 

 they are oval, then pear-shaped, and at last become of a violin-shape. 

 At the same time, they rise more and more from the surface of the 

 Area pellucida, so as to form ridges of higher elevation, with a deeper 

 groove between them; and the summits of these ridges tend to 

 approach one another, and gradually unite, so as to convert the groove 

 into a tube. It is within this, that the Cerebro-spinal Axis is after- 

 wards formed ; the brain being developed in the anterior dilated por- 

 tion, and the spinal cord in the posterior more contracted part. The 



Fig. 141. 



The germ and surrounding parts, from a more advanced Uterine Ovum :b, blastoderma, or germinal 

 membrane; a g, area germinativa; c, cephalic extremity of the germ; v, first indications of vertebrae; q, cau- 

 dal extremity. 



former remains unclosed much longer than the latter. The tube 

 within which the neural axis is thus enclosed, and which is entirely 

 composed of nucleated cells (Fig. 16), is termed the Chorda dorsalis ; 

 and it retains its embryonic type in many of the lower Fishes, which 

 never possess a true vertebral column. The elements of the vertebrae 



