458 



OF GENERATION AND DEVELOPMENT. 



are developed in a fibrous membrane, with which the chorda dorsalis 

 subsequently becomes invested ; the neural arches being the parts first 

 formed. 



813. During the progress of this change, another very important one 

 is taking place, which has reference to the nutrition of the embryo 

 during its further development. This is the formation of vessels in the 

 substance of the germinal membrane ; which vessels serve to take up 

 the nourishment supplied by the yolk, as well as that derived from the 

 chorion externally, and to convey it through the tissues of the embryo. 

 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. 142). This gradually 

 extends itself, until the vessels spread over the whole of the germinal 

 membrane. The vessels are probably formed by the coalescence of the 

 original cells of the layer ; and the first blood-discs which they contain 

 seem to originate in the nuclei of these cells. This network of vessels 

 serves to receive the nutritious matter contained in the yolk-bag, and 

 to convey it to the embryo ; but the act of absorption seems to be per- 

 formed here as elsewhere, by cells, a layer of which always intervenes 

 between the vascular layer and the yolk itself. These cells probably 

 correspond in function with those of the villi of the intestinal canal in 

 the adult ( 243) ; as the vessels of the yolk-bag, or temporary diges- 



Fig. 142. 



m Vascular area of Fowl's egg, at the beginning of the third day of incubation ;~a, a, yolk; b, b, b, b, venous 

 sinus bounding the area, c, aorta; d, punctum saliens, or incipient heart; e, e, area pellucida;/,/, arteries 

 of the vascular area; g, g, veins; h, eye. 



tive cavity, represent those of the alimentary canal, to be afterwards 

 developed from a portion of it. The vessels of the yolk-bag terminate 

 in two large trunks, which enter the embryo at the point that after- 

 wards becomes the umbilicus, and which are known as the Omphalo- 

 Mesenteric, Meseraic, or Vitelline vessels (Fig. 146, <?, r). The first 

 movement of fluid takes place towards the embryo ; and this may be 

 witnessed before any distinct heart is evolved. 



