VASCULAR SYSTEM. 



697 



While the heart or propelling organ is being formed within the 

 body of the embryo, the greater number of the primitive blood-vessels 

 are developed in the peripheral part of the blastoderm in the vascular 

 and transparent areas, and comparatively few arise in the embryo ; 

 these last consisting at first only of the two vessels, the primitive double 

 aorta, which carry the blood from the heart to the arteries distributed 

 in the perijiheral area, and the corresponding venous trunks which 

 return the blood from the area to the centre of the circulation. These 

 primitive vessels become afterwards the omphalo-mesenteric arteries 

 and veins of the yolk-sac. 



Fig. 509. — Diagrammatic Outlines op the Heaut and Peimitive Vessels op the 

 Embryo Chick as seen from below and enlarged. 



a, soon after the first establisliment of tlie circulation ; B, c, at a somewhat later 

 period ; 1, 1, the veins retiiruing from the vascular area ; 2, 3, 4, the heart, now iu the 

 form of a notched tulje ; 5, 5, (upper) the two primitive aortic arches ; 5, 5, (lower) 

 the primitive double aorta ; A, the single or united aorta ; 5', 5', the continuation of the 

 double aortaj beyond the origin of the large omphalo-mesenteric arteries, 6, 6. 



The first rudiment of the heart consists of an elongated tubular 

 contractile chamber hollowed out of a mass of mesoblastic cells in 

 front of the reflection of the cephalic fold into the flat part of the blasto- 

 derm. This tube is divided into two at its anterior and posterior 

 extremities, and perhaps it is originally entirely double. Posteriorly 

 the heart-chamber receives the nascent blood from the entering venous 

 channel on each side, and anteriorly it opens into two arterial vessels, 

 which passing one on each side of the primitive pharyngeal cavity, 

 and turning backwards below the protovertebral plates, form the two 

 primitive aortte before mentioned, from each of which by a sudden bend 

 outwards, as observed in birds, the omphalo-mesenteric arteries pass off 

 into the vascular area. There is, however, some difference in the 

 number and form of these arteries in birds and mammals, but in all 

 of them the first circulation begins in a similar vascular area, 

 and among the earliest veins formed is a circular or terminal sinus 



