DEVELOPMENT OF THE EMBRYO. 715 



tube an indication is soon perceived of a division into vertebrae, of which the 

 plates just mentioned are the incipient arches (Fig. 26, Plate II.). Towards 

 the anterior extremity, however, the dorsal lamina? do not at once close in ; 

 and the large cells, in which the great divisions of the Encephalon originate 

 ( 358), may be seen between them. From the Dorsal Lamina on either side, 

 a prolongation passes outwards and then downwards, forming what is known 

 as the ventral lamina; in this are developed the Ribs and the transverse pro- 

 cesses of the Vertebrae ; and the two have the same tendency to meet on the 

 median line, and thus to close in the abdominal cavity, which the dorsal 

 lamina? have to inclose the spinal cord. At the same time the layers of the 

 Germinal Membrane, which lie beyond the extremities of the Embryo, are 

 folded in, so as to make a depression on the yolk ; and their folded margins 

 gradually approach one another under the abdomen. In these two modes, a 

 cavity is formed beneath the Embryonic mass, which is separated from the 

 general cavity of the Yolk by the folds just described; but these still leave a 

 passage which, in the Bird, remains of considerable size until a much later 

 period, but which, in the Mammiferous Ovum, is soon obliterated. For the 

 sac which contains the yolk, and from which the abdominal cavity is pinched 

 off (as it were) at a very early period, is destined, in the Mammiferous ani- 

 mal, to be entirely cast away ; the purpose which it has to serve being one of 

 a very temporary character. 



938. Whilst these new structures are being produced, a very remarkable 

 change is taking place in that part of the Serous lamina, which surrounds the 

 Area Pellucida. This rises up on either side in two folds ; and these gradually 

 approach one another, at last meeting in the space between the general en- 

 velope and the embryo, and thus forming an additional investment to the 

 latter. As each fold contains two layers of membrane, a double envelope 

 is thus formed ; of this, the outer lamina adheres to the general envelope ; 

 whilst the inner remains as a distinct sac, to which the name of Amnion is 

 given. (See Figs. 284, 285, and 286.) This takes place during the third 

 day in the Chick ; the period at which it occurs in the Human Ovum is diffi- 

 cult to be ascertained, owing to the small number of normal specimens which 

 have come under observation at a sufficiently early stage. During the same 



Fig. 283. Fig. 284. 



Plan of early uterine Ovum. Within the Diagram of ovum at later stage ; the digestive ca- 



external ring, or zona pellucida, are the serous vny beginning to be separated from the yolk-sac, 



lamina, a; the yolk, b; and the incipient em- and the ainnion beginning to be formed; a, chorion; 



bryo, c. b, yolk-sac ; c, embryo ; d, and e, folds of the serous 



layer rising up to form the Amnion. 



period, a very important provision for the future support of the Embryo begins 

 to be made; by the development of Blood-vessels and the formation of Blood. 

 Hitherto, the Embryonic structure has been nourished by direct absorption of 



