252 TRANSACTIONS OF THE CANADIAN INSTITUTE. [YoL. II. 



At about the eleventh day the ventral portion of the mesoblastic plate 

 on each side consists of two layers of cells formin,^ the visceral and 

 parietal portion of the plate. These layers are closely applied to the 

 entoblast and ectoblast respectively, but not at first to each other, for 

 evidences of a slit-like space between them which represents a persistent 

 part of the primitive body cavity, can be very well seen at this 

 date. This slit quickly disappears through the growth of the adjacent 

 parts and the consequent pressure exercised on the mesoblastic cells. 

 The latter are, at first, more or less rounded in outline but the pressure 

 exerted on them gives them a somewhat flattened appearance, except at 

 the lower, extreme margin where the visceral and parietal layers become 

 connected, the cells of the visceral layer here retaining, to a considerable 

 extent, their original shape. 



This part of the mesoblast seems to possess a greater capacity for 

 proliferation than the more dorsally placed portions of the ventral half. 

 The proliferation is limited chiefly to the cells at the extremity of the 

 plate and to those immediately above this belonging to the visceral 

 layer. The latter at the point in question is, about the twelfth day, 

 formed of two or more series of cells, those constituting the most 

 internal layer becoming very much flattened and like, in this respect, the 

 cells of the single layer of the parietal portion. The cells placed 

 between are obviously in the position occupied previously by the slit- 

 like space, the more ventrally placed portion of the primary body cavity 

 and as they undergo division more frequently than the other cells, they 

 cause a still greater flattening of the remaining cells of the visceral layer 

 and of those of the parietal portion, with the result that these resemble 

 fully formed endothelial cells. In a transverse section of the larva at about 

 the thirteenth day, taken a short distance behind the developing heart, 

 the cells first described lie in two large masses one on each of the ventro- 

 lateral margins of the entoblast in which depressions exist to contain 

 the masses of cells. The depressions are lined by the flattened endothe- 

 lial elements derived from the visceral layer which are now recognisable 

 with difficulty, and covered externally by similarly flattened endothelial 

 cells derived from the parietal layer. The visceral and parietal layers 

 above this are still at this time formed each of only one layer of cells 

 more or less flattened. The cells constituting the masses described are 

 the haematoblasts, while the depressions in the yolk or entoblast consti- 

 tute the site of the subintestinal veins. 



As the subintestinal veins are followed backwards they are seen to ap- 

 proach, with the mesoblast plates, more and more the middle of the line 

 of the ventral side of the yolk and where the mesoblastic plates from 



