VASCULOGENESIS IN THE CAT 49 



In the embryo of eight somites (fig. 26) a moderate advance has 

 been made in organization and considerable changes in topog- 

 raphy have been effected. The most striking at first glance is 

 the increase in the internal limiting sulcus incident to the for- 

 mation of the foregut. But it will also be observed that the heart 

 has been carried forward and is now much nearer the blind ex- 

 tremity of the foregut than in the four somite embryo. The first 

 somite is also at a position cephalad to that of the younger embryo. 

 This might be taken to imply an addition of somites at the cranial 

 end of the series, a most difficult point to establish in the absence 

 of fixed landmarks and in the shifting topography of this region. 

 But this assumption would not explain the advance of the heart. 

 The increase of the mesoderm dorsad, and the formation of the 

 foregut seem to call for a rearrangement of this layer and may 

 possibly account for the shortening actually observed, though 

 this naturally does not exclude the possibility of the production 

 of somites from the head mesoderm. 



The aortic angiocysts are confluent from the first arch to the 

 fourth somite, and mesal to them are mesenchyme cells, many 

 but not all of which are adherent to their walls. The heart is 

 multilocular. The omphalomesenteric vein now has a continu- 

 ous lumen and has effected several small connections with the 

 omphalomesenteric plexus, which is now extensive. Amid its 

 channels are scattered mesenchyme and vasofactive cells and 

 numerous projections from the mesoderm, the tips of which on 

 reaching the entoderm spread out, form horizontal processes, 

 and partially enclose the endothelial channels (fig. 27). Sub- 

 sequently these projections are resolved into mesenchyme in 

 which the vessels are imbedded. 



In the embryo of twelve somites (Columbia 'Collection No. 

 547) the dorsal aorta has acquired a continuous lumen from the 

 first arch to the region of the primitive streak. Caudad it di- 

 minishes in diameter and terminates in connection with scattered 

 angiocysts and vasofactive cells, which farther caudad give place 

 to mesenchyme (fig. 13). The first arch is double, one arm pass- 

 ing laterad, one cephalad to the lateral cul de sac of the foregut. 

 The ventral aorta is endothelial and at the sides of the gut is 



MEMOIR NO. 3 



