52 Vertebrate Embryology 



with whose inner layer they fuse (Fig. 22). 

 The two lamellae of entoblast that make up 

 the pouches are at first in contact with each 

 other, so that there is no actual cleft between 

 them (Fig. 22, HM, HC.\) ; but at about the 

 time of the opening of the mouth the lamellae 



FIG. 19. HALF SECTIONS IN THE TRANSVERSE PLANE 



OF A TADPOLE, IO MM. LONG (LEFT HALF) AND OF A 

 TADPOLE 12 MM. LONG (RIGHT HALF). X 35- (After 



Marshall.) 



B F, fore-brain. O Z>, outer wall of optic cup (pigment layer 

 of adult retina) ; O C, inner wall of optic cup (remainder of adult 

 retina). O Z., lens, attached to epiblast in younger tadpole, but 

 forming a hollow vesicle at the later stage. TP. pharynx. O. 

 sucker. [G. H. F.] 



separate from each other to form the actual 

 clefts, all of which open to the exterior except 

 the hyomandibular pair, which recede from 

 the ectoblast and persist, for a time, as a 

 pair of diverticulae from the front part of the 

 pharynx. 



