THE BLOOD-VAS< II.AR SYSTEM. 



127 







embryo, and which is known as the "vascular area." So that the first blood- 

 vessels are developed outside the body of the embryo. Some of the cells of which 

 the vascular area is composed arrange themselves in cords, the cords forming a 

 network. Fluid begins to collect in the interior of the cords, forcing apart the 

 cells of Avhich they are composed, and converting them into canals, some of the 

 cells collecting here and there into groups adherent to the walls of the canals 

 and projecting into their lumen. These are the so-called "blood-islands" 

 (Fig. 96. f). and the cells which compose them separate later on and become the 

 embryonic blood-corpuscles. The blood-vessels early extend in toward the em- 

 bryo from the vascular area, the new vessels arising as bud- or spur-like out- 

 growths from those already existing. Eventually, the vasifactive process reaches 

 the embryo and the developing vessels come into contact and communicate with 

 the heart, which by this time has 

 been formed and is already pul- 

 sating before the vessels reach it. 



The earliest embryonic red 

 blood-corpuscles are all nucleated 

 and are more properly termed 

 blood-cells, true blood-corpuscles, 

 wliich in all the mammalia are 

 non-nucleated, making their ap- 

 pearance about the second month 

 of development and gradually re- 

 placing the embryonic blood-cells. 

 The origin of the corpuscles is 

 somewhat uncertain ; some em- 

 bryologists believe them to be 

 formed from the blood-cells by the 

 extrusion of the nuclei of the 

 latter, while others maintain that 

 they are special formations devel- 

 oping in the protoplasm of the red blood-cells and being thus from the beginning 

 non-nucleated. In later life the formation of red corpuscles seems to occur in 

 the marrow of the bones. The white corpuscles or leucocytes appear very early 

 in development, but their exact origin is not known ; probably they arise from the 

 mesoblastic tissue outside the blood-vessels and migrate into their interior, The 

 vitelline circulation commences about the fifteenth day and lasts till the fifth 

 week. When fully established it is carried on as follows : Proceeding from 

 the tubular heart are two arteries, the first aortic arteries (Fig. 97), which 

 unite at some distance from the heart into a single artery. This runs down in 

 front of the primitive vertebrae and behind the walls of the intestinal cavity, 

 and again divides into two primitive aortce or vertebral arteries, and these give 

 off five or six omphalo-mesenterie arteries, which ramify in that part of the blas- 

 toderm which surrounds the developing body of the embryo, and which is known 

 as the vascular area. They terminate peripherally in a circular vessel the 

 terminal sinus. This vessel surrounds the vascular portion of the germinal area, 

 but does not extend up to the anterior end of the embryo. It terminates on 

 either side in a vein called the omphalo-mesenterie. The two omphalo-mesenteric 

 veins open into the opposite extremity of the heart to that from which the arte- 

 ries proceeded. 



2. The Placental Circulation. As the umbilical vesicle diminishes, the allan- 

 tois and the placenta develop in the manner already indicated. When the um- 

 bilical vesicle disappears the latter becomes the only source of nutrition for the 

 embryo. The allantois carries with it to the placenta two arteries, derived from 

 branches of the primitive aorta, and two veins ; these vessels become much 

 enlarged as the placental circulation is established, but subsequently one of 

 the veins disappears, and in the later stages of uterine life the circulation is 



FIG. 96. A portion of the vascular area of a chick embryo, 

 a. Blood-vessels forming a network. 6. Meshes of the net 

 work. c. Blood-islands. (From Kolliker.) 



