DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 47 



sides the formation of the maxillary process, is in the appearance of 

 the appendages (aa and pa) ; they have the characteristic shape of 

 the rudimentary vertebrate appendage, though the anterior pair 

 seem to point in an unusual direction at this stage and to be slightly 

 more developed than the posterior. The curious, anteriorly directed 

 heart (lit) is, perhaps, somewhat abnormal. The umbilical stalk 

 (u) is comparatively narrow and, like the allantois, was cut off close 

 to the body. 



Transverse sections of an embryo of this stage are represented in 

 figures ija-g, drawn under a lower magnification than were any of 

 the preceding figures. 



Figure lya is in the region of the pharynx, and passes through 

 the forebrain (fb ) and posterior part of the hindbrain (lib). In the 

 thick walls of both of these structures histological differentiation has 

 begun, so that even under low power an inner granular and an outer 

 clear zone may be distinguished. Under greater magnification the 

 presence of short fibers may be made out among the cells. The 

 cerebral hemispheres (ch) are well-marked structures, their asym- 

 metry 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 con- 

 nection 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 (hi) is now a solid 

 mass, of the usual type for vertebrate embryos, its front or outer 

 wall being a scarcely discernible line. The hindbrain (lib) has the 

 usual form for that region and does not differ particularly from 

 what was noted in earlier stages except in the histological differ- 

 entiation 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 

 ( pli) forms an irregular cavity connected with the exterior on the 

 left by a gill cleft (g) and by another slit which is simply the ante- 

 rior margin of the stomodaeum. On the right neither of these 

 openings are in the plane of the figure, though the gill cleft (hyo- 

 mandibular), 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 notochord (nt) and 

 the floor of the brain, is noticeable from its being very closely packed 

 with corpuscles, so that at first glance, under low magnification, it 

 looks more like a nerve than a blood-vessel. 



Figure \yb is also through the pharyngeal region, a short distance 

 behind the preceding section. The growth of the cerebral hemi- 



