DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 57 



which separated at first the two halves of the heart became 

 absorbed, the endothelium lining of each of the originally sepa- 

 rate vessels would remain complete, dividing the cavity of the 

 heart into two parts. The partition in the central line is, Jiow- 

 ever, soon absorbed. 



The account given above chiefly differs from that of Remak 

 by not supposing that the mesoblast-cells which form the heart 

 are in any way split off from the wall of the alimentary canal. 



There can be no doubt that His is wrong in supposing that 

 the heart originates from the mesoblast of the splanchnopleure 

 and somatopleure uniting to form its walls, thus leaving a cavity 

 between them in the centre. The heart is undoubtedly formed 

 out of the mesoblast of the splanchnopleure only. 



Afanassiev's observations are nearer to the truth, but there 

 are some points in which he has misinterpreted his sections. 



Sections PI. 2, figs. 10 and 11, explain what I have just said 

 about the origin of the heart. Immediately around the noto- 

 chord the mesoblast is not split, but a very little way outside it 

 is seen to be split into two parts so and sp ; the former of these 

 follows the epiblast, and together with it forms the somatopleure, 

 which has hardly begun to be folded at the line where the sec- 

 tions are taken. The latter (sp) forms with the hypoblast (liy} 

 the splanchnopleure, and thus has become folded in to form 

 the walls of the alimentary canal (d). In fig. 11 the folds have . 

 not united in the central line, but in fig. 10 they have so united. 

 In fig. n, where the mesoblast, still following the hypoblast, 

 turns back to assume its normal direction, it is seen to be 

 thickened and to have become split, so that a cavity (of) (of 

 the omphalomeseraic vein) is formed in it on each side, lined by 

 endothelium. 



In the section immediately behind section fig. 11 the meso- 

 blast was thickened, but had not become split. 



In fig. 10 the hypoblast folds are seen to have united in the 

 centre, so as to form a completely closed digestive canal (d) ; the 

 folds of the mesoblast have also united, so that there is only a 

 single cavity in the heart (hz), lined, as was the case with the 

 omphalomeseraic veins, by endothelium. 



In conclusion, I have to thank Dr Foster for his assistance 

 and suggestions throughout the investigations which have formed 



