ORIGIN OF BLOOD AND ENDOTHELIUM 297 



When the plasma is flowing in a closed system within the em- 

 bryo, it is still running as a wandering stream through lacunae 

 and sinuses on the yolk. This probably explains why the 

 blood cells reproduce for so long a time on the yolk-sac while no 

 such reproduction is taking place in the well formed vessels of 

 the embryo. 



It is difficult to determine the exact moment, when, or place 

 at which the first blood cells get into the circulation. This 

 probably varies even among embryos of the same species. Zieg- 

 ler ('88) thinks, however, that just beyond the lateral plates in 

 the plasma filled spaces of the yolk-sac which lie between the 

 periblast and ectoderm, the first blood cells project into the 

 circulation. They are in the form of cell strings which later 

 connect the cardinal veins with the vascular yolk net. Swaen 

 and Brachet saw in trout embryos of eleven days in the region 

 of the fourteenth somite and posterior that the intermediate 

 cell mass spreads out laterally below the lateral plate and on 

 to the yolk surface. The cells thus came to lie above the yolk 

 syncytium and first attained their red color in this position. 

 These authors thus claiA that in the bony fish with a large yolk- 

 sac the haemoglobin free early blood cells through continued 

 contact with the yolk become transformed into erythrocytes. 



The experimental embryos considered in the present paper 

 demonstrate, however, that it is not at all necessary in such a 

 Teleost to have the erythroblasts reach the yolk-sac in order to 

 acquire their red haemoblobin condition. The tightly packed 

 erythroblasts within the intermediate cell mass of the embryo 

 develop perfectly and readily attain a normal red haemoglobin 

 color. 



Finally, comparing the processes of vessel and blood formation 

 in Teleosts with these processes in other vertebrate embryos, 

 we find no definite explanation for the formation of the inter- 

 mediate cell mass. In other embryos the blood is largely formed 

 upon the yolk. However, it must be recognized from recent 

 contributions that the formation of intra-embryonal blood is 

 much more extensive and important than has formerly been 



