

EMBRYO OF TWENTY-SEVEN SEGMENTS 57 



in the region of the mesencephalon. The hind-brain, as yet undivided, 

 equals the combined length of the other three vesicles. As the lens of 

 the eye invaginates, the wall of the optic vesicle folds inward, thus forming 

 a double-walled structure, the optic cup. The auditory placode has be- 

 come a sac, the otocyst, which overlies the hind-brain opposite the second 



Hind-brain 



Otic vesicle 



Branchial groove 3 



Amnionfold 



Atrium 



L. vitelline artery 

 Neural tube 



Primitive streak 

 atid tail bud 



Mid-brain 



Fore-brain 

 Optic vesicle 

 Lens vesicle 



rea pellucida 



. 56. Dorsal view of a fifty-hour chick embryo, stained and mounted in balsam. X 14. 



branchial groove and is still connected with the outer ectoderm, cut away 

 in Fig. 57. The rhomboidal sinus is still open at the caudal end of the 

 neural tube. 



Digestive Canal (Fig. 57). -In a reconstruction from the ventral side, 

 the digestive canal shows differentiation into three regions. Of these, 

 the fore-gut has been seen in earlier stages. A greater part of the mid-gut 

 has been cut away to show the underlying structures ; it is without a ven- 

 tral wall and overlies the yolk. Caudad, a small portal leads into the 

 hind-gut which is just beginning to evaginate into the tail fold. The 

 pharyngeal membrane now lies in a considerable cavity, the stomod&um, 



