232 H. D. Senior. 



which, it appears from the literature, have previously escaped notice. 

 Firstly: The endocardial cells do not descend in a hap-hazard 

 fashion; descent proceeds, in a perfectly orderly manner, continu- 

 ously from before backwards; the last cells to descend (*. e., those in 

 the region of the future aorta) are arrested, as it were, in the act of 

 descent to form the first part of the central aorta. Secondly : In the 

 region posterior to the aortic root descent of endocardial cells does 

 not occur at all ; the endocardium in this region is exclusively fur- 

 nished by cells descending in, and derived from, the region anterior 

 to the root of the aorta. 



The part of the portion moyenne, which at 15 somites was easily 

 recognizable in the region behind the (future) aortic root, extending 

 back as far as the middle of the otocyst( see Figs. 3E and 3F), has 

 disappeared long before the completion of the heart anlage, apparently 

 by blending with the adjacent somital mesoderm. It is still recog- 

 nizable at 18 somites (see right side of Fig. 4:E), but posterior to 

 this it has already disappeared or is very indefinite, see left side 

 of Fig. 4E (through their hyo-branchial anlage). At 22 somites 

 the portion moyenne, in this region, has entirely disappeared. 



Review of the evidence hearing on the relation of the endocardium 

 to the vascular endothelium of the head in general. 

 The development of the pericardial coelom, including in this term 

 the future myo-epicardium, may be looked upon as a subject prac- 

 tically complete in itself which can be considered independently of 

 that of other structures. The case of the endocardium is entirely 

 different; the fact alone that part (at least) of the ventral aorta 

 arises, in common with the endocardium, from the portion moyennne 

 is quite sufiicient to indicate that the endocardium cannot be con- 

 sidered as an independent structure. It is only reasonable to sup- 

 pose that the origin of the aorta is essentially similar throughout 

 the head, so that separation of the aorta from the endocardium, in 

 this connection, would be artificial and, therefore, not conductive to 

 a clear conception of the origin and relations of the latter. During 

 the study of the process of formation of the heart anlage several 

 facts bearing on the origin of the vascular endothelium became 



