ORIGIN OF BLOOD AND ENDOTHELIUM 255 



It may then be concluded from a study of the hving embryos 

 with a circulation and others without a circulation, that in the 

 normal ordinary individuals as well as in those having their blood 

 flow prevented, the origin and formation of blood in the bony 

 fish occurs as follows: The chief source of origin of the erythro- 

 blasts is that so fully described by previous investigators as the 

 intermediate cell mass, la masse intermediare. This mass, 

 according to Felix ('97), Swaen and Brachet ('99, '01) and others, 

 arises from the median portions of the two lateral mesodermal 

 plates, primary seiten-platten. These bi-lateral masses migrate 

 towards the middle line and there fuse to form the intermediate 

 cell mass or blood string. In the living embryo this very "m- 

 portant mass of blood cells is readily demonstrated. It is usually 

 median in position, but in many cases, as illustrated above, it 

 may be double or bi-lateral, at least in its anterior portion. 

 This bilateral arrangement may possibly be the result of a failure 

 of the blood forming portions of the two lateral plates to move 

 to the middle line and fuse to form a Stammvene, in other words, 

 a type of arrest. 



The second seat of differentiation of red blood cells which is 

 distinctly shown in living embryos is to be found on the yolk- 

 sac in the posterior and ventral region where numerous typical 

 blood islands form and develop. All recent investigators of the 

 development of the blood in Teleosts have denied the develop- 

 ment of blood on the yolk-sac. Most of their investigations 

 have been on the eggs of the trout, and it may be that in this 

 group there are no blood islands. But in Fundulus we seem to 

 have a transitional condition in which the yolk-sac islands have 

 not been firmly incorporated within the intermediate cell mass 

 but still remain out or wander out upon the yolk. A.t any rate, 

 we must conclude that there is a secondary seat of red blood 

 formation in Fundulus embryos, and that in life it presents the 

 typical appearance of yolk-sac blood islands. 



From a study of the living embryos, it is apparently impos- 

 sible to determine whether all cells of these blood islands are 

 only erythroblasts or of mixed types. This is, however, readily 

 ascertained by a careful study of sections. 



