I have obtained no evidence enabling me to determine whether Dr Klein is correct in stating that the cells of the mesoblast in the interior of the heart become converted partly into blood-corpuscles and partly into a cellular lining forming the endothelium of the heart, in the same way that the blood-vessels in the rest of the blastoderm are formed. But I should be inclined to think that it is very probable--certainly more probable than that the cavity of the heart is formed by a process of splitting taking place. Where I have used the word "absorption" in speaking of the formation of the cavity of the heart, I must be understood as implying that certain of the interior cells become converted into the endothelium, while others either form the plasma or become blood-corpuscles. The originally double formation of the hinder part of the heart probably explains Dr Afanassiev's statement (_Bullétin de l'Académ. Impériale de St Petersb._, tom. xiii, pp. 321-335), that he finds the endothelium of the heart originally dividing its interior into two halves; for when the partition of the mesoblast which separated at first the two halves of the heart became absorbed, the endothelium lining of each of the originally separate vessels would remain complete, dividing the cavity of the heart into two parts. The partition in the central line is, however, 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 Pl. 2, figs. 10 and 11, explain what I have just said about the origin of the heart. Immediately around the notochord 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 sections are taken. The latter (_sp_) forms with the hypoblast (_hy_) 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. 11, 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.