458 JOHN LEWIS BREMER 



arise directly as an extension of the mesothelial cords, without the 

 process of delamination. Any future cavity in these cords would 

 be potentially a part of the coelom. 



If these interpretations of the sections are correct, true endo- 

 thelium may arise in two ways from the mesothelium. Certain 

 appearances make it at least probable that blood corpuscles may 

 also be a product of the same tissue. In the yolk-sac blood-islands 

 it has long been agreed that both endothelium and corpuscles 

 come from the same anlages. In the body-stalk net of the Minot 

 embryo, which is not connected with the yolk-sac net, there are a 

 very few blood-corpuscles, or at least free cells within endothelial 

 cavities. In the Grosser embryo, in which the connection of the 

 two nets is apparently solid, there are also a very few corpuscles. 

 In both of these embryos the yolk-sac corpuscles are limited to the 

 distal pole; it seems certain, therefore, that the few blood-cor- 

 puscles in the body-stalk vessels must have developed in the net 

 itself. The blood-islands already mentioned in the body-stalks of 

 these two embrycs are either not connected with the other vessels, 

 or are so connected only by apparently solid strands (fgs. 1 and 2) ; 

 they will probably later supply their quota of corpuscles, but do 

 not seem to account for the few already present. 



In the embryo of Professor Meyer, already described as show- 

 ing mesothelial cords still connecting the blood-vessels with the 

 coelomic wall, the blood-corpuscles are differentially stained, and 

 easily distinguishable from other cells. They are found not only 

 in the blood-vessels, but also in the mesothelial cords, of which 

 they seem to form a part. Similarly in the drawing of the Grosser 

 embryo (fig. 7) there are, in and connected with the mesothelium, 

 cells whose nuclei resemble those of the three cells floating free in 

 the cavity. Though these cells are not of the type of corpuscle 

 which one would expect at such an early age, cells of the same 

 character are found occasionally in the yolk-sac vessels of both the 

 Grosser and the Minot embryos, and from their position I consider 

 it probable that they are blood-corpuscles. We should, then, 

 credit this mesothelium with the additional power of forming 

 occasional corpuscles without the mediation of blood-islands. 



