NO. II DIGESTIVE CANAL OF AMERICAN ALLIGATOR REESE 3 



of this stage. The mouth has now broken through, putting the 

 wide pharynx into comnnuiication with the exterior ; probably the 

 mouth opening is formed at about the time of the opening of the 

 first gill cleft. 



Figure 4A represents a transverse section through the head of an 

 embryo of the approximate age of the one just described ; it passes 

 through both forebrain. fb, and hindbrain, hh ; through the extreme 

 edge of the optic vesicles, ov, and through the anterior end of the 

 notochord. nt. It is just cephalad to the anterior end of the pharynx 

 and to the hypophysis. The chief purpose in showing this section 

 is to represent the two large head-cavities, he. The origin of these 

 cavities may be discussed at a later time. They are irregularly oval 

 in cross section, and extend in an antero-posterior direction for a 

 distance about equal to their long axis as seen in cross section. The 

 two cavities project towards each other in the middle line, and are 

 almost in contact with the notochord, in the region figured, but they 

 do not fuse at any point. These two head-cavities are the only ones 

 to be seen, in this animal, unless the small evaginations from their 

 walls represent other cavities fused with these. Their walls are thin 

 but distinct, and consist of a single layer of cells. These cells are 

 completely filled with their large, round nuclei, so that the wall has 

 the appearance, under higher magnification than is used in this 

 figure, of a band of closely strung, round beads. 



Figure 413 represents the eighteenth section caudad to the one just 

 described. It passes through that region of the enteron, ph, which 

 may be called the pre-oral gut, since it lies cephalad to the now open 

 mouth. Owing to the plane of the section the upper angle of the first 

 gill cleft, g', is seen on the left, although this would not naturally 

 have been expected in a section through the pre-oral gut. The 

 evagination to form the hypophysis, p, is seen against the floor of 

 the forebrain, fb. The wall of this region of the enteron is com- 

 paratively thin, and consists of not more than two layers of com- 

 pactly arranged cells with round nuclei. 



Figure 4c is about forty sections caudad to the one just described. 

 It passes through the mouth, seen as a vertical opening between the 

 two mandibular arches, md. The hyomandibular cleft, g', the only 

 one which opens to the exterior in this embryo, is very wide, and 

 may be traced through a number of sections ; in this section the 

 opening is seen only on the left. The pharynx, ph, is very wide; 

 as it is followed caudad its ventral opening is gradually closed by 

 the approach of the two mandibular folds. The dorsal wall of this 

 region of the pharynx is very thin, consisting of a single layer of 

 flat cells with round nuclei ; while the ventral wall, leading through 



