EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



275 



irregular form of the embryo the sections through the otocyst are not symmetrical. 

 The present section shows the opening of the left otocyst, Ot.s, and a closed section 

 of the right otocyst, Ot. d. At its lower inner edge the outer boundary of the wall 

 of the otocyst is indistinct, this appearance being due to the union of the cells of 

 the acoustic ganglion with the wall of the otocyst. The section also passes through 

 the first gill cleft, cl. i, of the right side, and shows very distinctly indeed the 



Epen. 



h.b. 



Ot. 



Nth. 



h.b. 



Ot.d. 



Re. 



Ao. D. 



FIG. 153. SECTION OF CHICK EMBRYO WITH 

 ABOUT TWENTY-EIGHT SEGMENTS. TRANS- 

 VERSE SERIES 92, SECTION 73. 



Ao, Aorta. EC, Ectoderm. F.b, Fore-brain. h.b, 

 Hind-brain, tries, Mesoderm. Nch, Notochord. 

 Of, Otocyst. Ve, Vein. X 5 diams. 



FJG. 154. SECTION OF CHICK EMBRYO WITH 

 ABOUT TWENTY-EIGHT SEGMENTS. TRANS- 

 VERSE SERIES 92, SECTION 83. 



Ao.D, Descending aorta. Ao.2, Second aortic arch. 

 card, Anterior cardinal vein. cl.pl, Closing 

 plate, cl.i, First gill pouch. EC, Ectoderm. 

 Epen, Roof of hind-brain, f.b, Fore-brain, h.b, 

 Hind-brain, mes, Mesoderm. nch, Notochord. 

 Op, Optic vesicle. Ot.d, Right otocyst. Ot.s, 

 Left otocyst. PA, Pharynx. X 5 diams. 



closing plate, cl. pi, which is formed by a fusion of the ectodermal and entodermal 

 cells. On the opposite side of the section the same cleft is imperfectly shown. On 

 the posterior side of the cleft is the second aortic arch, Ao. 2, and on the ante- 

 rior side of the cleft, extending toward the fore-brain, /. b, appear the sections of 

 the two descending aortae, Ao. D. The part shown in this section is that which 

 connects the dorsal ends of the first and second aortic arches. In the 



