The Development of the Heart in Shad. 237 



ference in behaviour of the cells in question does not necessarily 

 depend on the nature of the cells themselves, but appears to be due 

 to conditions occurring in the middle region which are not found 

 in the posterior. It may be said that the portion moyenne appears 

 to consist in the main of cells which will later form vascular endo- 

 thelium, but it is difficult or impossible to show that in the anterior 

 region it includes all of these, or that in the middle region it does 

 not include more. In other words, although the portion moyenne 

 forms a well defined group of cells which is of great assistance in 

 following the movements of the endocardium, it does not appear, in 

 itself, to be a structure of real morphological importance. 



Greil, in his recent paper on the origin of the blood and blood-ves- 

 sels, '08, traces the origin of the endocardium in Ceratodus, and in 

 some amphibia and selachii, from two sources which he distinguishes 

 as the Angioscleroblast and Angioha3moblast. Greil does not refer to 

 teleosts in this connection, but his statement gives additional in- 

 terest to the description by Boeke, '03, of a two-fold origin of the 

 endocardium in Mursena. Boeke describes the major part of the 

 endocardium as arising from cells developed in the head-region, but 

 traces the origin of some of the cells lining the venous end of the 

 heart from the region of the closing blastopore (the latter cells would 

 seem to correspond to those described by Greil as emanating from the 

 hsemangioblast) . I have looked carefully for cells corresponding 

 to those described by Boeke as arising from the region of the blasto- 

 pore without result, and believe that any cells which may migrate 

 forward from this region in shad must be arrested posterior to the 

 junction of the first body somite with the head. 



It is intended to re-investigate the origin of the aorta in the head, 

 and to look for additional evidence regarding the origin of the jugular 

 veins, and afterward to study the origin of the vascular endothelium 

 of the body vessels. 



PERIOD 2. LASTING UNTIL RIIYTIIIMICAL CONTRACTION BEGINS 

 IN THE PARTIALLY FORl^fED HEART-TUBE. 



Between the stages of 30 and 32 somites, the heart anlage has 

 undergone little change; at 33 somites the beginning of progress 



