236 H. D. Senior. 



and middle regions suggest the origin of the dorsal aorta from the 

 portion moyenne, any attempt to prove this supposition is met with 

 the difficulty that the aortic cells are known to differentiate m situ. 

 To trace a few undifferentiated aortic cells in their migration through 

 mesodermal cells which they closely resemble would scarcely be pos- 

 sible by ordinary cmbryological methods. 



In the posterior region the cells adjacent to the medial borders 

 of the lateral plates are not isolated, as in the anterior and middle 

 regions, to form portion moyenne. Fig. 9 shows that the aorta in 

 the posterior region is situate immediately dorsal to the lateral margin 

 of the entodemial pharynx, and, in the posterior part of this region, 

 just dorsal to the medial margin of the lateral plate as well (sections 

 from this part of the posterior region are shown in Figs. 3G and 5G ; 

 7G is slightly anterior to it). The aorta is thus placed exactly iri 

 the line of the mesodermal cells adjacent to the medial borders of 

 the lateral plates apparently arises directly from them. 



In the foregoing notes an attempt has been made to bring together 

 some evidence bearing on the development of the vascular endothelium 

 of the head in order to arrive at a conception of the nature and rela- 

 tions of the endocardium. The evidence in question appears to 

 justify the statement that the endocardium in shad arises from meso- 

 dermal cells which are found, after differentiation of the lateral 

 plates, to be placed in the region bordering on the medial margins 

 of the latter; further, that the mesodermal cells in this situation 

 appear to be given up exclusively to the formation of vascular endo- 

 thelial cells of which the endocardium only forms a part. 



A few words may be added by way of re-examination of the 

 ''Portion moyenne du mesoblaste" of Swaen and Brachct. In the 

 anterior pericardial region the cells which later form the endocardium 

 must necessarily be separated from the somital portion of the meso- 

 derm, for the former eventually take their place as endocardium 

 ventral to the lateral plates, while the latter retains its position 

 dorsal to them. In the middle and posterior pericardial regions, 

 separation of the cells bordering on the lateral plates is not a neces- 

 sity, for neither region produces endocardium. ^Nevertheless, separa- 

 tion occurs in the middle region but not in the posterior; the dif- 



