The Devolopnioiit of \\w Heart in Shad. 227 



the hyoid cleft. The anterior prominence is not strictly pharyngeal ; 

 it represents the posterior limit of the head-fold ectoderm/^ which, 

 growing in from either side, blends with the anterior extremity of 

 the closing entodermal pharynx. Inasmuch as it has been found 

 difficult or impossible to distinguish the line of union between ecto- 

 derm and entoderm after blending has occurred, no attempt has 

 been made to indicate this in Figs. 4, 5, 6 and 7 ; there is no doubt, 

 however, that, although the lateral prominences consist of ectoderm, 

 the axial region (as far as shown in the figures) is truly pharyngeal. 

 At 18 somites the anterior part of the portion moyenne is des- 

 cending on either side, betw^een the lateral plates and the, now closing, 

 pharynx to gain a position ventral to these structures ; the longitudinal 

 area over which descent is occurring ("descent area") is indicated 

 by the space betw^een the arrows in the diagram. Fig. 4A is a sec- 

 tion from this region. Sections from behind the descent area (Figs. 

 4B and 4C) show that the portion moyenne, which can be seen 

 in them dorsal to the lateral plate, is not descending ; descent is 

 prevented, apparently, by the close contact between lateral plate and 

 pharyngeal entoderm. It will be seen that the portion moyenne is 



"The "optic region" of the head (anterior to the parts reconstrneted) is 

 separated from the yolk by a double layer of ectoderm which grows in, in this 

 situation, from the basal ectoderm around the anterior periphery of the head. 

 This ectoderm, which performs several functions, corresponds, in teleosts, to the 

 head-fold of other vertebrates (see Froriep, W)) . The formation of head-fold 

 from this double layer of ectoderm occurs literally, in shad, only as far back 

 as the hypophysial region, anterior to which no gut-entoderm occurs. There is 

 a region, extending from some point (approximately) ventral to the hypo- 

 physis back to a point slightly posterior to the tip of the notochord. throughoat 

 which the ingrowing ectoderm encounters the anterior extremity of the phar- 

 yngeal entoderm ; the ectoderm in this region, although its origin is intimately 

 connected with that of the head-fold, has an entirely different subsequent his- 

 tory, briefly indicated as follows : Until the head of the embryo begins to arise 

 from the yolk the double-layered ectoderm of the region in question forms a 

 bond of union between the anterior end of the pharynx and the surface of the 

 embryo. As the head rises from the yolk the layers of the head-fold ectoderm 

 proper become separated to cover the contiguous portions of the head and 

 yolk. Shortly before perforation in the oral plate occiu's, the anterior cul-de- 

 sac of the pharynx becomes widely dilated, and the ectoderm connecting the 

 lateral margins of the anterior end of the entodermal pharynx with the basal 

 ectoderm of the surface undergoes, very rapid, disintegration. 



