The Development of the Alhgator 309 



ence of short fibers may be made out among the 

 cells. The cerebral hemispheres (ch) are well- 

 marked structures, their asymmetry being of course 

 due to the obliquity of the section. Only one 

 eye is cut by the plane of the section, and this one 

 shows no connection with the forebrain. The 

 outer wall of the optic cup (oc) is so thin that 

 under this magnification it can scarcely be seen 

 as a dark line surrounding the retinal wall. The 

 lens (In) is now a solid mass, of the usual type 

 for vertebrate embryos, its front or outer wall 

 being a scarcely discernible line. The hindbrain 

 (hb) has the usual form for that region and does not 

 differ particularly from what was noted in earlier 

 stages except in the histological differentiation 

 that has already been mentioned. As with the eye, 

 it is only on the right side that the auditory vesicle 

 (o) is shown. It shows some differentiation, but 

 not so much as would be seen were it cut in another 

 region. In the center of the section the pharynx 

 (ph) forms an irregular cavity connected with the 

 exterior on the left by a gill cleft (g) and by an- 

 other slit which is simply the anterior margin of the 

 stomodasum. On the right neither of these open- 

 ings is in the plane of the figure, though the gill 

 cleft (hyomandibular) , which lies close to the 

 auditory vesicle, is almost an open passage. A 

 few small blood-vessels are scattered through the 

 section; one of these (bv), lying between the noto- 

 chord {7it) and the floor of the brain, is noticeable 



