WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 79 



cells in the cat by Schulte (1914), forecasting, as it were, the long series of vitelline 

 arteries which is to follow. There are in addition, also at the posterior end of the 

 aorta, indications of a very few short, mesially directed branches, not always closed 

 off from the wide intercellular spaces adjoining, which represent the first dorsal rami 

 of the aorta. The exact location of the vitelline artery can not be determined; con- 

 tinuing the segments backward would place it about the region of the sixth, the 

 future third cervical. Felix places the anterior connection with the aorta at the 

 seventh segment, in the stage of 5-6 somites, while in a 2.6 mm. embryo with 13-14 

 somites this connection has shifted back to the tenth segment on the right side and 

 beyond the twelfth on the left. 



On the left side certain minor differences are to be observed. In the region 

 of the somites the aorta is larger than on the right. This is followed by a short 

 stretch where the vessel is possibly interrupted but represented, at least in part, by 

 a solid chain of large cells. Its posterior end is enlarged like that on the right, but, 

 instead of being open, is occupied by a small blood island (fig. 12). The conditions 

 here must be essentially the same as those described by Miller and McWhorter in 

 the chick (1914), where the posterior third of the aorta on the operated side is repre- 

 sented by an elongated, cord-like blood island, whose cells "are identical with the 

 cells in the blood islands of the normal area opaca " (I.e., p. 209) . In this particular 

 case, as well as in our own, the diffusion, not to mention any circulation, must be 

 extremely tardy in this solid vessel. With the establishment of even very slender 

 connections, such as are present on the opposite side in both embryos, and the 

 ensuing rise in the rate of fluid interchange, we may expect a certain change in the 

 appearance of the blood islands such as are found in the umbilical veins of our own 

 embryo, or their gradual disappearance as well. The only formed elements which 

 we have found in the embryonic vessels, aside from the blood island just noted, are 

 a few cells in the caudal portion of the right aorta, both conditions recalling the 

 cell clusters described in this vessel at much later stages in other animals (Emmel, 

 1916; Jordan, 1917). As regards branches on the left side, very little can be made 

 out. The same angioblastic cells are present as on the right side, but they are 

 rather less conspicuous. If there is a vitelline artery already laid down here it is 

 very indefinite and located at the caudal end of the aorta in the region occupied by 

 the blood island. Both aortae lie very close to the entoderm; the wide space inter- 

 vening between these vessels and the mesoderm is bridged by great numbers of 

 cell processes of varying size and shape and is also comparatively free from nuclei. 

 This space is rather wider than that between the mesoderm and ectoderm. The 

 surface of the mesodermic mass toward the entoderm is much more irregular and 

 there are more isolated mesenchymal elements here than underneath the ectoderm. 

 From the mesoderm there are undoubtedly many cells migrating out, where they 

 may very easily come into intimate relation with the aorta or assist in forming some 

 of its first branches (Schulte, 1914). 



It is not only in the splanchnopleure that vasculogenesis is in progress, but also, 

 although to a much less extent, in the somatopleure as well. There is to be found on 

 the somatopleure on the left side, very near its reflection to form the amnion, a 



