224 H. D. Senior. 



which is now in progress of diiferentiation from behind forwards, 

 is not yet clearly distinguishable at the site of Fig, 3A (q.v.) and 

 here the portion moyenne (although it is later well developed, see 

 Fig. 4A) is not clearly defined. 



The distinction of the portion moyenne from the somital portion 

 of the head mesoderm depends on an actual separation of cells from 

 the lateral region of the somite; or (perhaps more correctly) on 

 an isolation of the cells situated between the somite and the lateral 

 plate. The position of the portion moyenne, where portion moyenne 

 occurs, bears a definite relation to the lateral margin of the entodermal 

 pharynx. In the region of the entodermal pouches the pharynx is 

 wide and the portion moyenne is always situated lateral to its lateral 

 margin (see Figs. 3B, 3C, 3E and 3F). Posterior to the region 

 of the entodermal pouches the entodermal pharynx is much reduced 

 in width, and here the portion moyenne does not occur at all, for the 

 lateral plate is in contact with the somite (see Figs. 3G and 5G). 

 The transition between the wide and narrow portions of the ento- 

 dermal pharynx occurs, somewhat abruptly, on a level with the 

 middle of the otocyst. In the region of the entodermal pouches, the 

 prominent lateral margin of the pharynx tends to insinuate itself 

 between the somital mesoderm and the portion moyenne. Posterior 

 to the entodermal-pouch region the margin of the narrow entodermal 

 pharynx tends to remain altogether ventral to the somital mesoderm 

 (see Figs. 3G and 5G). 



Swaen and Brachet suggest that the portion moyenne is separated 

 from the somital mesoderm under the combined inflvience of the 

 prominent margin of the entodermal pharynx on the one hand, and 

 of the ectoderm and the anterior part of the otocyst on the other; 

 the conditions found in shad are quite in accordance with this view. 

 The appearances in Fig. 3F clearly suggests the influence of the wide 

 hyoid pouch and of the otocyst in causing separation of the portion 

 moyenne. In Figs. 3B, 30 and 3E the ectoderm would appear to 

 be of little assistance in causing separation of the portion moyenne, 

 but it is probable that the ectoderm is normally in contact with the 

 mesoderm and that the separation of ectoderm from mesoderm, seen 

 in the sections, is artificial and mainly due to the action of the 

 fixative. 



