The Development of the AUigator 321 



except that the tip of the wall of one of the cerebral 

 hemispheres {ch) is cut. The left nasal chamber 

 {n) is shown: it will be noted again in the following 

 section. The eye on the right side shows no 

 remarkable features; its lens (In) is large and lies 

 well back of the lips of the optic cup, which may 

 now be called the iris {ir). A thin layer of meso- 

 blast has pushed in between the lens and the 

 superficial ectoderm to form the cornea, and the 

 outer wall of the optic cup is now distinctly pig- 

 mented. The inner wall of the optic cup is be- 

 ginning to differentiate into the retinal elements. 

 The eye on the left side is cut farther from its 

 central region and has a very different appearance 

 from the eye just described. This unusual appear- 

 ance is due to the fact that the section passed 

 through the choroid fissure, which is very large and 

 seems to be formed by the pushing in of the walls 

 of the cup and not by a mere cleft in these walls. 

 This fissure is hardly noticeable in the stage pre- 

 ceding the present, and in a stage slightly older it has 

 disappeared ; so that it would seem to be a very tran- 

 sient structure. It apparently is formed at about 

 the time that the optic stalk, as such, disappears. 

 It is in the region of the choroid fissure, if not through 

 it, that the optic nerve {on) enters the eye. Through 

 the fissure also enters a vascular tuft of mesoblast 

 {pt) which may be seen projecting into the optic 

 cup after the disappearance of the fissure. This 

 loop of blood-vessels is doubtless the pecten. 



