DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 195 



splanchnic mesoblast is the cavity of the future heart. In this 

 space lie a number of cells, H, variable in size, but mostly spherical 

 in shape ; they have large nuclei, and are richly studded with yolk 

 granules. 



These are clearly the cells which are destined at a slightly later 

 stage to form the cardiac endothelium. Concerning their origin, we 

 have always found them to be perfectly distinct from the mesoblast, 

 while in several cases they are in very close relation with the 

 hypoblast. In some instances their connection with the hypoblast is 

 a very intimate one, and we have noticed endocardial cells apparently 

 in the act of separating off from the outer ends of the hypoblast 

 cells. In this region there is, as Eabl has pointed out, a slight 

 median longitudinal groove on the floor of the pharynx, continuous 

 anteriorly with the thyroid depression. This groove causes a slight 

 keel-like projection on the under surface of the pharynx, and it is 

 in connection with this keel that we have found the appearance of 

 budding off of endocardial cells from the hypoblast to be best marked. 

 The section drawn in Fig. 1 is taken through the hinder end of this 

 groove, which is here hardly recognisable. 



The hinder end of the heart is from the first continuous with the 

 vitelline veins that form on the surface of the yolk. The endothelial 

 cells of this hinder part are at this stage larger and more spherical 

 than those further forward, and we are inclined to think that in 

 this region the cardiac endothelial cells are formed in the manner 

 described by Schwink. 



We have found no evidence, however, of a shifting forward of the 

 endothelial cells, such as must occur if all the endothelial cells arise 

 in the region of the liver. On the other hand, we have, as explained 

 above, obtained evidence that the endothelial cells of the anterior 

 part of the heart arise directly from the hypoblast of the ventral 

 wall of the pharynx. 



I loncerning the hypoblastic origin of the endothelium in the frog, 

 while fairly assured of its actual occurrence, we think it would be a 

 mistake to attempt to assign to it any great morphological signifi- 

 cance. In many respects the development of the frog, especially in 

 the early stages, gives evidence of secondary modification of an 

 extensive kind, it will be sufficient to call to mind the two-layered 

 condition of the epiblast, and the consequent peculiarities in the 



