FORMATION OF FIRST BLOOLVESSELS. 933 



downwards and forwards of the lateral portions of the blastodermic vesicle. 

 In Fig. 338, the mode of the development of the abdominal cavity is seen, 

 its parietes being formed by the somatopleure, composed of the epiblast with 

 some mesoblast (6/1), whilst within it the deep groove of the intestines (<l) 

 appears, the walls of which or splanchnopleure are composed of two laminae, 

 the mesoblast (df), and the hypoblast (d), passing into the corresponding 

 lamina of the blastodermic vesicle, which already form the vitelliue sac. 

 The incipient intestine is maintained in its place by the mesentery, which 

 begins to be formed out of a prolongation of the mass lying in front of the 

 chorda dorsalis, in which lie the now symmetrical aorta (so) and the cardinal 

 vein (vc), and which is obviously only a thickened portion of the connecting 

 baud between the parietal lamina and the intestinal fibrous layer. 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 for- 

 mation of Blood. Hitherto, the embryonic structure has been nourished by 

 direct absorption of the alimentary materials supplied to it by the yolk ; 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 the vascular area or that part of the germinal 

 membrane which immedialely surrounds the embryo. 1 In their earliest 

 stage of development these vessels appear as tivo arcus aortce, proceeding 

 from the anterior part of the heart, which after a short course in the for- 

 ward direction, bend backwards to unite in a single, short, symmetrical 

 aortic tube, from which again two branches almost immediately arise, the 

 arfcrice vertebrates posteriores, or primitive aortae. These, lying beneath the 

 chorda dorsalis, extend to the posterior part of the body of the embryo, di- 

 viding ultimately into the omphalo-mesaraic, or Vitelliue vessels (Fig. 340, 

 E.Oj.A, L.Of.A ; see also Plate II, Fig. 13) which form a close network, 

 bounded by a circular channel termed the Vena or Sinus terminalis. This 

 network or vascular area soon extends itself, and the vessels finally spread 

 over the whole of the membrane that contains the yolk. At the anterior 

 part of the embryo the two extremities of the circular channel form the 

 Vence omphalo-mesaraicce, which discharge their contents into the back part 

 of the heart. At this period the Yolk-sac is entirely separated in the Mam- 

 malia, by a constriction of the portion which is continuous with the abdomen 

 of the embryo (Fig. 343, b) ; 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 discernible. The two first veins, then, 

 that are developed, are the Venae omphalo-mesaraicse, which belong not to 

 the body of the embryo itself, but to the germinal area, and open by a short 

 tube common to both into the posterior extremity of the Heart (Fig. 339, 

 I, o?/0. Subsequently by the extension of their branches over the yolk-sac 

 they become the vitelline vessels, still opening by two veins into the heart, 

 and effecting the absorption of the yolk. With the formation of the kites- 

 tine, however, the vessel of the right-side disappears, the left alone remaining 



1 According to His (loc. cit., p. 95), Wolff, .and Pander, in the chick a peripherical 

 vessel around the germinal area, and others in the area opaca, first appear, whilst at, 

 various points small colored masses, surrounded by a membrane or blood-islands are 

 developed. Both the vessels and the blood proceed directly from the white or germ- 

 yolk The formation of the vessels precedes that of the heart, and is quite inde- 

 pendent of it. 



