THE DEVELOPMENT OF THE VASCULAR SYSTEM. 193 



products of the embryo for deposition in the maternal circulation, the waste 

 in the lower forms (reptiles and birds) being deposited in the allantois. 

 The blood carries from the placenta the food materials derived from the 

 maternal circulation, the food in the lower forms being taken from the yolk 

 sac and conveyed to the embryo by the vitelline vessels. 



Principles of Vasculogenesis. Upon the thesis that tissues in general 

 must receive materials which they build up into their own substances and 

 must discharge the products of their activities, the vascular channels of 

 the body can be considered as structural expressions of this functional 

 necessity. For instance, a muscle which acts must receive materials to 

 compensate it for its loss and must discharge the waste products that result 

 from its action, and the blood vessels are peculiarly adapted to these func- 

 tions. The lymph vessels, too, similar in structure to the blood vessels, 

 although efferent relative to the tissues, play their part in conveying the 

 products of metabolism. 



Much controversy has arisen over the actual genesis, or origin, of blood 

 vessels and lymphatics, and as yet the opposing views have not been recon- 

 ciled. In brief there are two views: One that with a few exceptions every 

 vessel in the body develops as a sprout from another vessel, that is, the 

 endothelium arises from preexisting endothelium by proliferation of its own 

 cells; the other that vessels in general arise in situ, that is, the lumen of a 

 vessel represents an intercellular tissue space^or several such spaces, whose 

 bordering cells have been transformed into the characteristic endothelial 

 cells, and as a corollary, the continuity of a given vessel results from the 

 union of such spaces. According to the latter view, the whole vascular 

 system represents intercellular tissue spaces which, with their lining of 

 flattened cells, have united to form a set of continuous channels. 



In the case of either view it is recognized that the first vessels appear 

 in the opaque area of the blastoderm. Here the blood islands originate as 

 clusters of cells amidst the mesoderm, differentiating from mesenchymal 

 elements in close approximation to the entoderm (Fig. 157). The superficial 

 cells of the clusters are then transformed into flat cells placed edge to edge 

 to form the endothelial wall of a primitive blood space. These blood 

 spaces join one another and thus form a net-work of channels. From this 

 point in development the two views diverge. 



The evidence adduced in favor of either theory is too great in volume 

 to set down here. The advocates of the theory of sprouting of the endo- 

 thelium lay stress upon the evidence of injected specimens. By injecting 

 developing blood vessels at successive stages it is found that the vascular 

 field gradually becomes larger, and the inference is that the individual 

 channels are extending farther and farther from the focus of origin through 



