68 CHAELES R. STOCKARD 







either side of the pharynx, begin to migrate towards the mid- 

 dle line. The two masses fuse into one, spaces are developed in 

 the mass and finally the endothelial lining of the heart is differ- 

 entiated out of this group of cells. The lamellae of the side 

 plate become separated and the space between them gives rise 

 to the pericardial cavity. Oellacher (73), Wenckebach ('86), 

 Henneguy ('88), and Sobotta ('94) have all described the origin 

 of the heart in Teleosts in much the same way. 



Several of these investigators, Wenckebach, Swaen and 

 Brachet and others, have called attention to a small mass of cells 

 derived from the heart anlage which comes to lie beneath and 

 outside the heart endothelium. This mass of cells has been 

 claimed to wander away from below the pericardium and later 

 to give rise to vessels and blood on the yolk-sac. In the non- 

 circulating Fundulus embryos, however, neither vessels nor 

 blood are formed on the extreme anterior portions of the yolk-sac. 

 I have seen nothing in my studies which would indicate that any 

 cells left over from the heart formation had wandered upon the 

 yolk or given rise to blood cells or vascular endothelium. 



Swaen and Brachet are alone in showing that the heart cells 

 are definitely continuous with the intermediate cell mass of the 

 the trunk mesoderm. 



Many early workers on the fish embryo have claimed, as has 

 been done for most vertebrae classes, that the heart lining arises 

 from endoderm. The weight of evidence at the present time is 

 so overwhelmingly against such a view that it warrants only 

 a passing mention. Again, however, it must be realized that 

 in the separation of the mesoderm from the endoderm it is 

 possible that some future mesoderm cells may be left behind 

 not cleanly separated. These cells might later isolate themselves 

 from the endoderm to form vessels or blood. It nevertheless 

 seems generally true that all blood forming cells are at one time 

 in development contained within the mesodermal portion of the 

 embryo. Gregory ('02) came to the conclusion that the endo- 

 derm and mesoderm could be traced to an indifferent cell mass 

 mesentoderm in certain Teleosts, and according to his view, 

 there is no way to say from which germ layer the heart endothe- 



