ORIGIN OF BLOOD AND ENDOTHELIUM 67 



Anterior of the first somite in the unsegmented mesoblast of 

 the head this division or pinching away also takes place. Thus 

 the intermediate cell mass of the body becomes continuous with 

 a definite lamella of the head. This well definied topographical 

 portion of the embryonic mesoblast, the intermediate cell mass 

 and cell lamella, is, according to Swaen and Brachet, the only 

 material which gives rise to the heart, the chief vessels and the 

 blood in the embryo. This description by Swaen and Brachet 

 ('01) agrees very closely with that formerly given by Felix ('97), 

 except that Felix disagrees in not deriving the aorta from the 

 intermediate cell mass but from the sclerotoms. 



The observations made upon the intermediate cell mass in 

 Fundulus are in close accord with this summary. But no at- 

 tempt has been made to solve the detailed question as to whether 

 the aorta is derived from the intermediate mass or from the. 

 sclerotoms. It would seem that this vessel might arise froin 

 either source and still be formed from practically identical cells. 

 Since in the separation of the primary lateral plate from the 

 somite it is easily conceivable that some cells which generally 

 accompany the primary lateral plate might be left as part of 

 the lateral portion of the somite. This lateral portion of the 

 somite is the part which later separates as the sclerotom so that 

 the cells destined to form the aortic endothelium might occur 

 equally well within the intermediate cell mass or within the 

 sclerotom. Their location might vary among different species 

 or even among individuals, and yet these aortic cells would be 

 derived from the same genetic source. 



Swaen and Brachet also indicate the head mesoblast as sepa- 

 rated into three portions: the intermediate cell mass close to 

 the top of the pharynx, the lateral plate split into two lamellae 

 and the general head mesoblast close around the brain. The 

 intermediate cell mass is more intimately connected with the 

 splanchnic layer of the lateral plate. The pharynx widens in 

 forming the gill pouches which continue to grow dorsally and 

 finally separate the intermediate cell mass into two portions, 

 one part thus comes to lie ventral of the pharynx and the other 

 part dorsal. The ventral portions, at first solid masses below 



