1890-91.] AMPHIBIA BLOOD STUDIES. 253 



each side unite in the middle line, the veins form a single channel, 

 til a point immediately in front of the anus is reached. In its 

 course backwards the vessel is filled with cells closely packed and derived, 

 in the same manner as those forward are, from the visceral layer of the 

 mesoblast, although it is more difficult to exclude here the participation 

 of the parietal layer in the formation of the haimatoblasts. The meso- 

 blastic plates again diverge at the anus and the venous trunk bifurcates, 

 a branch running separately on each side of the cloacal cavity, the cells 

 contained in them becoming less in number till, for lack of them, it is im 

 possible to follow the veins any distance behind the anus. 



When these veins and the cellular elements in them have attained the 

 development described the heart is formed and beats. At first it contains 

 no organized elements, the force of the beat being, apparently, exercised 

 on what would appear to be serum. About the fifteenth or sixteenth day 

 cellular elements in every respect like those found in the subintestinal 

 veins are found in large numbers in the heart cavity and as the 

 subintestinal veins are almost empty it is clear that the haematoblasts 

 are derived from this source. It is, in fact, easy in series of sagittal 

 sections of larvae of the fourteenth and fifteenth days to see the detach- 

 ment of the haematoblasts in the anterior portions of the subintestinal 

 veins and their arrival in the heart cavity. 



The haematoblasts are derived from this source alone. All the other 

 vessels of the body have a different origin, that is, they are not formed by 

 solid columns of cells exerting a pressure on the immediately adjacent 

 mesoblastic elements, but rather by the extension of the subintestinal 

 vessels and of the cavities of the heart. In Amblystoma larvae therefore 

 the haematoblasts are of mesoblastic origin alone and they are not in- 

 creased in numbers by additions from the yolk elements or entoblast. 



At first they are large, not differing from mesoblast cells in any- 

 thing except their somewhat spherical shape. They contain in their 

 cytoplasma a large number of yolk spherules which obscure more or less 

 the nucleus. The latter is somewhat irregular, often amoeboid in outline 

 and richer, apparently, in chromatin than the ordinary mesoblastic cells 

 of the same stage of development. To this greater richness in chromatin 

 may be attributed the more abundant proliferation of these cells, for one 

 can see that cell division is more frequent in them than in the neigh- 

 boring cells. As the quantity of yolk spherules is limited, the repeated 

 division, probably accompanied by a digestive action on the part of the 

 cell on the spherules, produces a form of haematoblast (Fig. i6 and 17 

 a and b) in which the yolk spherules are few and in which nuclear 

 chromatin is very abundant. It is in this stage that one finds the 



