1 86 Vertebrate Embryology 



exterior; while in the chick the entire thick- 

 ness of the epithelium is involved in the 



Fig. 62. — Transverse section through the ante- 

 rior REGION OF A CHICK EMBRYO OF ABOUT TWENTY- 

 EIGHT SEGMENTS. (After Minot.) 



^(?.Z)., descending aorta. ^4(7. 2. second aortic arch. crtr(/., ante- 

 rior cardinal vein, cl.fil.^ closing plate, cl.l^ first gill pouch. Ec, 

 ectoderm. Epen.y roof of hind-brain, f.b.^ fore-brain, k.b., hind- 

 brain, wify., mesoderm. «r/?., notochord. C?/., optic vesicle. Ot.d., 

 right otocyst (ear vesicle). C^'.J., left otocyst. Z'^., pharynx. 



formation of the auditory pits, and after 

 the pits become closed to form the vesicles 



