14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



are thin, the epithelium consisting of but a single layer of more or 

 less flattened cells. 



Figure 7 represents a reconstruction of the enteron of an embryo 

 of 42 mm. crown-rump length. Because of the body flexure and 

 large size of the embryo the head was amputated, in the plane a-b, 

 and cut sagitally, while the body was cut transversely in the direction 

 shown by the section planes. In the present figure the outline of the 

 embryo, including the eye, appendages, and umbilical stalk, is shown 

 by fine dotted lines ; the outlines of the lungs and liver are shown by 

 heavier, broken lines ; while the outlines of the enteron proper and 

 the trachea are shown in solid lines, filled in which fine stippling. 

 For the sake of simplicity only one lung and one bronchus are shown. 



Since the head has now quite a reptilian form, the oral cavity, m, 

 has more or less of the adult outline. A transverse groove near the 

 anterior end of the lower jaw marks off the tongue, tn; and the 

 rudiments of teeth are seen but not shown in the figure because of 

 the low magnification used. 



The pharynx, ph, is a very extensive cavity that is sharply sepa- 

 rated from the mouth by a prominent transverse fold of skin, the 

 velum palitum, vp, just in front of the posterior nares, pn, and by a 

 less marked fold from the base of the tongue; it is these two valves 

 that enable the adult alligator to open its mouth under the surface 

 without getting water into the lungs. The mouth and pharynx are 

 lined at this stage with a thin, stratified epithelium, which consists of 

 a basal layer of rather tall columnar cells and one or two superficial 

 layers of flattened cells. The pharyngeal epithelium is rather thicker 

 than that of the oral cavity. 



In the embryo from which this reconstruction was made the phar- 

 ynx was in direct communication with neither the oesophagus nor the 

 trachea, though the separation in each case was by a mere membrane. 

 The trachea, ta, opens, except for this membrane, into the pharynx 

 a short distance back of the transverse, dorsal and ventral folds 

 mentioned above, and almost directly ventrad to the posterior nares. 

 The anterior end of the oesophagus, oe, is in contact with the 

 extreme postero-ventral wall of the pharynx. 



The trachea, which is already surrounded by distinct cartilaginous 

 rings, is long, and of about the same diameter throughout. In the 

 region of the anterior appendage, at the point marked X, it divides 

 into the two very short bronchi, which almost immediately open into 

 the lungs, hi. The lungs, whose structure will be shown in the 

 sections of this stage, are large, irregular bodies, extending about 

 equal distances cephalad and caudad to their openings into the 



