DEVELOPMENT OP THE EMBRYO. 991 



sufficiently early stage. During the same period, a very important provision for 

 the future support of the embryo begins to be made, by the development of 

 bloodvessels and the formation of blood. Hitherto, the embryonic structure 

 has been nourished by direct absorption of the alimentary materials supplied to 



Fig. 262. Fig. 263. 



Plan of early Uterine Ovum. Within the Diagram of Ovum at the commencement of the for- 



external ring, or zona pellucida, are the mation of the Amnion: a, chorion; b, yelk-sac; c, 



serous lamina, a ; the yelk, b ; and the in- embryo ; d, and e, folds of the serous layer rising up to 



cipient embryo, c. form the amnion. 



it by the yelk ; but its increasing size, and the necessity for a more free com- 

 munication between its parts than any structure consisting of cells alone can 

 permit, call for the development of vessels through which the nutritious fluid 

 may be conveyed. These vessels are first seen in that part of the Vascular lamina 

 of the germinal membrane ( 294) 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 (Plate II. Fig. 13). This gradually extends 

 itself, until the vessels spread over the whole of the membrane that contains the 

 yelk. The first blood-disks appear to be formed from the nuclei of the cells, 

 whose cavities have become continuous with each other to form the vessels 

 ( 149) j and from these, the subsequent blood-disks of the first series are pro- 

 bably generated. This network of bloodvessels serves the purpose of absorbing 

 the nutritious matter of the yelk, 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 yelk, and of con- 

 version into blood, probably continues as long as there is any alimentary mate- 

 rial left in the sac. 



999. The Yelk-sac is early separated in the Mammalia, by a constriction of 

 the portion which is continuous with the abdomen of the Embryo ; and it is 

 known from that time under the name of the Umbilical Vesicle (Plate I. Fig. 

 10, i). The communication, however, remains open for a time through the 

 vitelline duct ; and even after this has been cut off, the trunks which connect 

 the circulating system of the embryo with that of the vascular area are discern- 

 ible ; these are called Omphalo-Mesenteric, Meseraic, or Vitelline vessels (Figs. 

 266, 267, q, r). It was formerly believed that the nutrient matter of the yelk 

 passes directly through the vitelline duct into the (future) digestive ^cavity of 

 the embryo, and is from it absorbed into its structure ; but there can now be 

 little doubt, that the vitelline vessels are the real agents of its absorption, and 

 that they convey it, through the general circulating system, to the tissues in 

 process of formation. They correspond, in fact, to the Mesenteric veins of In- 

 vertebrated animals, which are the sole agents in the absorption of nutriment 

 from their digestive cavity ( 459) ; and the blastodermic vesicle is to be regarded 



