48 VASCULOGENESIS IN THE CAT 



The embryo of four somites (fig. 24) shows a very considerable 

 advance beyond this condition and illustrates the rapidity, al- 

 most abruptness, of the changes in vasculogenesis which have been 

 described by Huntington. 87 The heart which is now concealed 

 in the myocardium is composed of a series of endothelial vesi- 

 cles the walls of which are in contact and are often connected 

 with processes of the mesoderm. In general structure it is 

 not dissimilar to the segment of the omphalomesenteric vein 

 with which it is continuous caudad. Cephalad the line of the 

 ventral aorta is represented by plates of mesenchyme, vasofac- 

 tive cells, and small endothelial sacs. A large multilocular cyst, 

 showing evidences of concrescence in its septa, forms the cephalic 

 element of the dorsal aorta. Laterad it is connected with the 

 first arch which is already endothelial, and a similar connec- 

 tion is established with the juxta-neural line, i.e., the angiocysts 

 are in contact, but their lumina are still divided by numerous deli- 

 cate septa (fig. 25). A second endothelial sac in the aortic line 

 is situated just cephalad of the somites, ending near the first so- 

 mite in continuity with a flattened plate of mesenchyme cells. 



Corresponding to the other somites are plates of mesenchyme, 

 vasofactive cell masses with cleft-like lumina, and a few angio- 

 cysts. The omphalomesenteric plexus is composed of larger 

 vesicles which are now coalescing, but their lumina are not as 

 yet continuous with either the heart or the aorta. Between the 

 mesal vasculogenetic line (the dorsal aorta) and the lateral, which 

 comprises the ventral aorta, the heart and the omphalomesenteric 

 vein and plexus, are scattered mesenchyme and vasofactive 

 cells; cephalad these are more abundant and are now coalescing 

 to bridge the interval between the dorsal and the ventral aorta. 

 The first arch is endothelial and situated somewhat in front of the 

 point at which connection is established with the juxta-neural 

 line. Two other transverse strands are present, one largely 

 mesenchyme, one of vasofactive cells. 



87 Developmentaf relations between lymphatic amd haemal vascular channels. 

 Paper read at the XVII Internat. Med. Congress. London, August, 1913. To be 

 published in the Am. Jour. Anat. 



