716 OF REPRODUCTION. 



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 nutritious 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 net- 

 work, bounded by a circular channel, which is known under the name of the 

 Vascular Jlrea (Fig. 27, Plate II.). This gradually extends itself, until the 

 vessels spread over the whole of the membrane 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 ( 222) ; 

 and from these, all subsequent blood-discs are probably generated. This net- 

 work of blood-vessels serves the purposes 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 Yolk, and of conversion 

 into Blood, probably continues as long as there is any alimentary material left 

 in the sac. 



939. The Yolk-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. The com- 

 munication, however, remains open for a time through the constricted portion, 

 which is termed the Vitelline Duct ; and even after this has been cut off, the 

 trunks which connected the circulating system of the Embryo with that of the 

 Vascular Area, are still discernible; these are called Omphalo-Mesenteric, 

 Meseraic, or Vitelline vessels. It was formerly believed, that the nutrient 

 matter of the yolk 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 to the tissues in process of formation. 

 They do, in fact, correspond to the Mesenteric veins of Invertebrated animals, 

 which are the sole agents in the absorption of nutriment from their digestive 

 cavity ( 674) ; and the yolk-bag, as already remarked, is the temporary sto- 

 mach of the Embryo, remaining as the permanent stomach in the Radiated 

 tribes. Previously to the ninth day of incubation (in the Fowl's egg), a series 

 of folds are formed by the lining membrane of the yolk-bag, which project 

 into its cavity; these become gradually deeper and more crowded, as the bag 

 diminishes in size by the absorption of its contents. The Vitelline vessels, 

 that ramify upon the yolk-bag, send into these folds (or valvulse conniventes) 

 a series of inosculating loops, which immensely increase the extent of this 

 absorbing apparatus. But these minute vessels are not in immediate contact 

 with the yolk; for there intervenes between them a layer of nucleated cells, 

 which is easily washed away. It was from the colour of these, communicated 

 to the vessels beneath, that Haller termed the latter vasa lutea; when the 

 layer is removed, the vessels present their usual colour. There seems good 

 reason to believe, that these cells, like those of the Intestinal Villi in the adult 

 ( 672), are the real agents in the process of absorbing and assimilating the 

 nutritive matter of the yolk ; and that they deliver this up to the vessels, by 

 themselves undergoing rupture or dissolution, being replaced by new layers. 



940. The formation of the Heart takes place in the Vascular layer, beneath 

 the upper part of the Spinal Column; it at first appears as a mere cavity in its 

 substance, surrounded only by cells ; but its walls gradually acquire firmness 



