FOWL— THIRD DAY 



541 



cular cavity while upon each side and ventrally 1 there is seen a 

 localized thickening (olf) of the ectoderm: this is the dimple-like 

 rudiment of the olfactory organ. To return to the main part 

 of the section — there is seen in its centre the wide pharyngeal 

 space and on the embryo's left side the pharyngeal wall pro- 

 jects out to the ectoderm as an endodermal pocket — the rudiment 

 of the second visceral cleft (v.c.II). Immediately ventral to the 

 pharynx is the ventral aorta (v. A). On the left side of the embryo 

 the aortic root (a.? 1 ) is seen immediately dorsal to the pharynx, 

 while on the right side — the section not being accurately transverse — 

 a hoop-like aortic arch (a.a.III) is seen passing dorsalwards round 

 the side of the pharynx from ventral aorta to aortic root. The large 



v.A 



u.c.H 



Fig. 237e. 



-Transverse section passing through the second visceral cleft and the 

 olfactory rudiment. 



a.a.III, third aortic arch: a.c.v, anterior cardinal vein ; am, amnion; a.r, aortic root; f.am, false 

 amnion; f.b, forebrain ; h.h, hind brain; olf, olfactory rudiment ; ph, pharynx; v.A, ventral aorta; 

 v.c.II, second visceral cleft. 



vessel lying dorsal and slightly external to the aortic root (a.c.v) is the 

 anterior cardinal vein. Traced tailwards it is found to open into the 

 dorsal end of the duct of Cuvier. The neural tube (h.h) is seen to 

 have a thin roof and widely expanded lumen indicating that it is 

 now passing into the region of the hind brain. 



In tracing the series of sections further forwards it will be 

 realized that the front part of the head region is, owing to its 

 reflexed position, actually being traced in a morphologically tailward 

 direction. In the section figured (Fig. 237f) the reflexed portion of 

 the head is cut at the level of the eye rudiments (opt) which are 

 seen to be in the optic cup stage with the inner or retinal layer 



1 It will be realized from an inspection of the figure of the entire embryo that the 

 recurved part of the head is reversed in position. Its ventral side lies therefore in the 

 figure towards the right. 



