AORTA AND AORTIC ARCHES IN RABBITS 113 



presupposes that all vessels are injectable, and in fact the claim 

 is made that many collapsed vessels cannot be distinguished in 

 sections until opened and marked by the injection mass. While 

 in no way wishing to belittle the value of this method of research, 

 or to discourage the increase of the many beautiful and valuable 

 preparations obtained by its use, I still feel that its limitations, 

 as they are shown in this paper, should be pointed out. As 

 insisted on by His in 1900, and by many other authors (His' name 

 is used as that of the champion of this idea) , the first blood-vessels 

 on the yolk-sac and elsewhere are solid cords or strands of cells, 

 without lumen : or to use other words, the actual vessels are always 

 preceded by solid growths, which secondarily become hollow 

 and form vessels. These solid growths, for which I wish to pro- 

 pose the term 'angiobast cords,' usually take the form of nets, 

 which may persist until the separate strands are hollow, as shown 

 by the injections of Evans and others, or may, as I hope to show, 

 disappear in part without ever becoming injectable. The two 

 views are clearly shown by a comparison of two figures, one from 

 His ('00, 2, fig. 91), the other from Evans ('09, figs. 1, 2, 3) both 

 showing the caudal end of the aorta of a chick embryo; by the 

 injection method the capillary network is revealed, while His 

 represents a network of solid sprouts preceding the hollow vessels. 

 In this case, since an injection of these so-called solid sprouts would 

 give practically the same picture as would be obtained if they 

 were not seen and so left out of the drawing (the network being 

 similarly placed throughout) , we have no direct proof that the 

 sprouts are not potentially hollow, or in other words merely col- 

 lapsed; but in the development of the anterior part of the aorta 

 there are nets of solid angioblast cords present at an early stage, 

 parts of which have certainly never been shown by injections, 

 and may therefore, for the present at least, be considered solid. 

 Here and there in this solid network there are hollow spaces, or 

 true vessels, unconnected at first with one another and with the 

 lateral capillary net except by the solid angioblast cords, and 

 therefore not to be reached by any injection mass from this 

 lateral net; for such unconnected hollow spaces I suggest the 

 term 'angiocysts.' Thus the angioblast cords retain certain char- 



