DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 53 



ventricles (ch) and cuts the anterior edge of the pineal body (cpi). 

 The pineal body is very large and is directed backward instead of 

 forward, as is usually the case among the lower vertebrates (if the 

 alligator may be so classed). It is shown in figure 17a of a pre- 

 ceding stage and will be again shown in a sagittal section to be 

 described later. The same areas of more dense and less dense 

 mesoblast noted in the preceding figure are seen here. 



Figure 20c, though still in the head region, shows several features 

 that were not seen in the preceding figures. On the left of the hind- 

 brain (lib) the auditory vesicle (0), which is now considerably more 

 advanced than in earlier figures, is seen as a larger, flask-shaped 

 cavity and a smaller, round one. Between the larger cavity and 

 the hindbrain is the root of a cranial nerve (en), apparently the 

 eighth, since in another section it comes in close contact with the 

 wall of the larger part of the auditory vesicle just mentioned. On 

 the right side, ventral to the hindbrain, another and much larger 

 nerve (en) is seen. Nearly in the center of the figure is seen a 

 small, irregular, thick-walled cavity (p), this is the pituitary body, 

 and its connection with the roof of the pharynx may easily be made 

 out in another section. The mesoblast in this region of the sections 

 contains numerous large and small blood-vessels and exhibits certain 

 denser areas which probably represent the beginnings of the cranial 

 cartilages. No sign of the forebrain is seen (the plane of the section 

 passing in front of that region), 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 mesoblast 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 pigmented. The 

 inner wall of the optic cup is beginning 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 appearance is due to the fact that the sec- 

 tion 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 preceding the present, and in a stage slightly older it has 

 disappeared ; so that it would seem to be a very transient 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 



