DEVELOPMENT OF THE EMBRYO. 



613 



entirely cast away ; the purpose which it has to serve being one of a very 

 temporary character. 



761. 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 gradu- 

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

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

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

 thus formed ; of this, the outer lamina adheres to the general envelop ; whilst 

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

 (See Figs. 185, 186, and 187.) This takes place during the third day in the 

 Chick ; the period at which it occurs in the Human Ovum is difficult to be 

 ascertained, owing to the small number of normal specimens which have come 

 under observation at a sufficiently early period. 



Fig. 182. 



Fig. 183. 



Plan of early uterine Ovum. Within the 

 external ring, or zona pellucida, are the 

 serous amina, a; the yolk, b; and the inci- 

 pient embryo, c. (After Wagner.) 



Diagram of Ovum at later stage; the digestive cavity 

 beginning to be separated from the yolk-sac, and the 

 amnion beginning to be formed ; a, chorion ; b, yolk-sac ; 

 c, embryo ; d and e, folds of the serous layer rising up to 

 form the amnion. (After Wagner.) 



762. During the same period, a very important provision for the future sup- 

 port 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 the alimentary materials supplied to it by the 

 Yolk ; in the same manner as the simplest Cellular plant is developed at the 

 expense of the carbonic acid, moisture, &c., which it obtains for itself from the 

 surrounding elements. But its increasing size, and the necessity for a* more 

 free communication between its parts than any structure consisting of cells 

 alone can permit, call for the development of Vessels, through which the nutri- 

 tious fluid may be conveyed. These vessels are first seen in that part of the 

 Vascular lamina of the Germinal Membrane which immediately surrounds 

 the embryo ; and they form a network, bounded by a circular channel, which 

 is known under the name of the Vascular Area (Fig. 27, Plate II.]. This 

 gradually extends itself, until the vessels spread over the whole of the mem- 

 brane containing the yolk. The first blood-discs appear to be formed from 

 the nuclei of the cells, whose cavities have become continuous with each other 

 to form the vessels ( 575) ; and from these, all subsequent blood-discs are 

 probably generated. This network of blood-vessels serves the purpose of 

 absorbing the nutritious matter of the Yolk, and of conveying it towards the 

 embryonic structures, which are now in process of rapid development. The 

 first movement of the fluid is towards the embryo ; and this can be witnessed 

 before any distinct heart is evolved. The same process of absorption from the 

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