1 86 Vertebrate Embryology 



exterior; while in the chick the entire thick- 

 ness of the epithelium is involved in the 



h.b. 



Ot.d 



mes 



f.b. 



FIG. 62. TRANSVERSE SECTION THROUGH THE ANTE- 

 RIOR REGION OF A CHICK EMBRYO OF ABOUT TWENTY- 

 EIGHT SEGMENTS. (After Minot.) 



A o.D., descending aorta. A0.2. second aortic arch, card., ante- 

 rior cardinal vein, cl.pl. , closing plate, cl.l, first gill pouch. EC., 

 ectoderm. Epen., roof of hind-brain, f.b., fore-brain, h.b., hind- 

 brain, mes., mesoderm. nek., notochord. Op., optic vesicle. Ot.d., 

 right otocyst (ear vesicle). Ot.s., left otocyst. Ph., pharynx. 



formation of the auditory pits, and after 

 the pits become closed to form the vesicles 



