234 



II. D. Senior, 



In Fig. 7C the entodermal pharynx and pericardium are in contact, 

 and here cells descending from the portion moyenne have heen ar- 

 rested lateral to the line of contact between the two; reference to 

 Fig. 9 will show that these cells correspond in position to the second 

 (or oblique) part of the ventral aorta. This is not all; in Fig. 7C 

 (also 7B and YD) just dorsal to the entodermal pharynx there are 

 one or two cells on either side which differ markedly from the sur- 

 rounding mesodermal cells; the cells in question (which can be dis- 



Hyo-branchial 

 ectodermal pouch 



^Margin of Entodermal pharynx 

 Pharynx, lumen 



Fig. 9. — Diagram showing tlie course taken by tlie riglit aorta tlirougliout 

 the pericardial region of shad, stage 42 somites x 100 diams. 



The dorsal surface of the entodermal pharynx is shown on the right side, 

 and the dorsal surface of part of the lateral plate on the left. 



tinguished in practically every section used in the reconstruction) 

 are found (see Fig. 9) to invariably occupy the line of the dorsal 

 aorta and are, without doubt, aortic endothelium. It is hoped that 

 the position of the aortic cells has been sufficiently indicated in the 

 figures referred to; there are rarely more than two dorsal aortic 

 cells on each side of a section, the differentiation of these will be 

 illustrated in a future study of the aorta itself. 



