ORIGIN OF BLOOD AND ENDOTHELIUM 69 



Hum actually arises. A mixture of endoderm and mesoderm 

 cells gave rise to endocardium. 



The later development of the heart of the bony fish proceeds 

 much as in the case of other vertebrates, as has been carefully 

 described in detail by Senior ('09). The only point of interest 

 in the present discussion is the origin and significance of its en- 

 dothelial portions, and here Senior after a very thorough investi- 

 gation confirms in all general points the previous findings of 

 Swaen and Brachet. 



In Fundulus as in other Teleosts the heart endothelium par- 

 tially forms in loco but is also added to by wandering cells or 

 ingrowths of mesenchymal cells adjacently located. The venous 

 end of the heart leads directly down upon the yolk periblast, 

 and as was shown in several figures, this periblastic material 

 with huge amorphous nuclei may be at times drawn up into the 

 cavity of the heart. This would indicate that the venous end 

 was entirely free or not connected with any other vascular en- 

 dothelium. This condition is, no doubt, due to the absence of the 

 anterior yolk vessels which should in ordinary cases unite or fuse 

 with the end of the heart so as to establish a closed circulation. 



According to Swaen and Brachet in the region of the third 

 somite the intermediate cell mass forms only the aorta, while 

 caudad the aorta arises from the dorsal cells of the mass and the 

 great part of the mass forms the red blood corpuscles and the 

 venae cardinales. The endothelium of the cardinal veins finally 

 surrounds the blood cells, but before these cells are fully developed 

 or free, plasma has begun to flow in the aorta and other arteries. 



In pelagic forms hi which the egg is extremely small and devel- 

 ops very rapidly, the intermediate cell mass in the forward body 

 sections is very small, sometimes only seen between the somites. 

 This portion gives rise to the aorta. The cells are somewhat 

 more numerous in the middle and posterior sections, but they 

 never form a mass to the extent found in the larger demersal 

 eggs. At the tune of hatching the posterior cell strings form two 

 lateral longitudinal vessels from the beginning of the mesonephros 

 caudad to the anus. These two vessels, Swaen and Brachet 

 consider to be homologous to the unpaired median stem vein 



