NO. 



II DIGESTIVE CANAL OF AMERICAN ALLIGATOR REESE II 



angles of the clefts, g\ are composed of but a single layer of cells. 

 In the dorsal wall these cells are flattened, while in the ventral wall 

 they are more rounded. This difference in the shape of the cells 

 accounts for the slightly greater thickness of the floor over that of 

 the roof of the pharynx. The gill clefts no longer communicate with 

 the exterior. 



Figure 6c represents the caudal half of the embryo in the plane 

 475 of figure 6a. The section of the pharynx, ph, is here crescentic 

 in outline, and the pharyngeal walls, especially the floor, are some- 

 what thicker than in the more anterior section just described. 

 Lying a short distance dorsad to the pharynx are seen two small, 

 thick-walled openings, ty; these are the rudiments of the thymus 

 glands. They are here quite distinct from the enteron, and may be 

 traced through a large number of sections, being in some regions 

 solid and of a smaller diameter than in the present section. 



Figure 6d is in the region of the line 500 in figure 6a. The 

 thymus rudiments, ty, have about the same appearance as in the 

 preceding figure, except that they are som,ewhat larger. The 

 pharynx, ph, is much smaller than in the last section, and though 

 somewhat crescentic in outline, its convex side is dorsal instead of 

 ventral in position. The pharyngeal walls are here thicker, and 

 consist of two or three layers of cells, instead of the single layer of 

 more anterior sections. 



In the median plane the floor of the pharynx is pushed down, as a 

 solid tongue of cells, gs, the anterior edge of the glottis. Ventrad 

 and laterad to the glottis a crescentic condensation of mesoblast rep- 

 resents the beginning of the laryngeal cartilages, la. 



Two or three sections caudad to the one just described, the two 

 layers of which the tongue of cells from the floor of the pharynx is 

 composed separate slightly at the bottom to form a small cavity, the 

 trachea, ta; this condition is shown in figure 6e, which represents 

 part of a section through the plane 532 of figure 6a. 



The oesophagus, oe, is here a solid, crescentic mass of cells, the 

 lumen being completely obliterated. The dorsal part of the tongue 

 of cells, mentioned above, connects the ventral side of the oesophagus 

 with the trachea, like a sort of mesentery. Above the oesophagus, 

 on either side, is the thymus rudiment, ty, in this section practically a 

 solid mass of cells instead of a tube. The epithelium of the trachea 

 here consists of three or four layers of compactly arranged cells; 

 this epithelium is surrounded by a dense mass of mesoblast which is 

 responsible for the greater thickness of the trachea as seen in figure 

 6a. As has been said, the oesophagus here has no lumen, and when 

 examined under high magnification its walls are found to be com- 



