AORTA AND AORTIC ARCHES IN RABBITS 117 



this can be seen by noting the position of the somites in fig. 1. 

 Just lateral to the somite is the thin neck of mesoderm, the nephro- 

 tomy, which coming between two thicker portions, will be held 

 above the entoderm, to form the roof of a small longitudinal 

 canal; it is in this canal that the angioblast cords have room to 

 expand and become hollow (fig. 2, a). In the head region, a 

 similar though broader canal is provided by the uplifted lateral 

 edge of the medullary groove; and in this region the reconstruc- 

 tion shows that the hollow parts of the net are found more widely 

 distributed; in fact their chain follows forward the line of the 

 spreading edges of the medullary groove. Thus, as in the case 

 of the vertebral arteries, a longitudinal trunk is made by the anas- 

 tomosis of branches from transverse vessels. 



Another place where the blood-vessels have an opportunity 

 to develop freely is beneath the coelom. At this early stage the 

 coelom is almost exclusively represented by the amnio-cardiac 

 vesicles, which lie cephalad to the level of the somites. Here is 

 seen the development of the chain of spaces which is to become 

 the lateral heart; the extension caudally of this is limited by the 

 absence of the coelom. 



Here then, in this early stage of the embryo before the pharynx 

 has begun to be formed, we see a flat sheet of angioblast cords, 

 forming a network, lying between entoderm and mesoderm, de- 

 rived as an extension from the lateral extra-embryonic area. Its 

 growth is limited cephalad by the absence of mesoderm in the 

 proamnion; mesially by neural groove or somite. It occupies 

 two areas especially where further growth is possible, namely 

 under the nephrotome or the angle of the medullary groove, and 

 under the amnio-cardiac vesicle. 



Turning now to the next older embryo, one of six or seven seg- 

 ments, figs. 3 and 4, we can see that two changes have taken place ; 

 a portion of the net has vanished, and the remaining portions 

 include more hollow vessels, which are still, however, connected 

 by solid angioblast cords. The persistent parts of the net are 

 exactly those indicated in the last paragraph as occupying favor- 

 able positions: beneath the coelom lies the lateral heart, and 

 beneath the edge of the neural groove lies the dorsal aorta. This 



