Figure 7D represents a section through the plane 680 of figure 7.

The stomach, _i'_, which is cut through its middle region, is somewhat larger than in the preceding figures, though its walls have about the same character. Its outer walls are continuous, to a considerable extent, with the tissue of the surrounding body wall, especially in the region just caudad to the plane of the present section.

The duodenum, being cut through a double loop (see figure 7), is seen in two places, dorsally where it is cut through the edge of one loop, and ventrally where it is cut square across. In both sections the structure is the same, as might be expected, figure 7J. The surrounding mesoblast is differentiated into muscle fibers, figure 7J, _ml_, which form a fairly distinct layer; inside of this layer is a tall columnar epithelium, _ep'_, which is thrown into prominent folds. A thin layer of mesoblast, probably the submucosa, _sl_, lies beneath the epithelium and projects up into the folds. About ten or twelve folds are seen in any one section; only the larger ones are well seen in figure 7D.

Figure 7E shows a section through the plane 770 of figure 7. It is in the region of the umbilicus, _u_, and the extreme caudal end of the stomach which has been called the gizzard, _gz_. The small size of the gizzard is due to its being cut near its caudal margin. The enteron is here cut in no less than seven places: the reason for this will be evident on examination of the plane of the section as shown in figure 7.

Dorsal to the gizzard the section cuts the so-called caec.u.m, _ce_, a little nearer its anterior end than is shown in figure 7. The duodenum, _d_, is cut at five points, and has about the same structure as in the preceding figure. The character of the duodenal loops that causes the rather curious appearance of the present figure will be readily understood by reference to figure 7, though the reconstruction is not mathematically accurate. The ventral projection of the lower loops of the duodenum into the umbilicus is seen both in the present figure and in the reconstruction. The loop of the duodenum that, in the sections, is seen to lie directly ventrad to the gizzard, in the reconstruction is shown too much to the side of the latter organ. The descending loops of the duodenum are cut in such a way that the surrounding mesoblast forms a continuous ma.s.s of tissue.

Figure 7F represents a section through the plane 901 of figure 7. The section pa.s.ses through the kidneys, _k_, the edge of one posterior appendage, _pa_, the large intestine, _il_, and two regions of the small intestine, _i_.

The large intestine is here a thick walled, cylindrical structure, _il_, hanging from a thin mesentery, _ms_, in the much reduced body cavity.

The layers of its wall are much more fully differentiated than in the more anterior regions of the enteron. The epithelium is here stratified instead of simple columnar, and the folds into which it is thrown are broader and less numerous than in the duodenum above described.

Ventrad to the large intestine, and almost in contact with it, is seen the allantois, _al_, whose general outline was noted in connection with figure 7. It is an irregular structure, consisting of a very thin outer layer of mesoderm, lined with a single layer of flattened epithelial cells.

Lying at a considerable distance ventrad to the main body of the section, are seen the two sections of the small intestine, _i_, surrounded by irregular strands of tissue from the umbilicus. The structure of these two intestinal loops is about the same as in the more anterior region described above.

Figure 7G, the last of this series, represents a section through the cloaca, caudad to the urinary openings, in the plane 1060 of figure 7.

The epithelium of the cloaca is, of course, simply a continuation of that of the surface of the body, somewhat thickened, perhaps, in the deeper regions.

The intromittent organ, _io_, which projects cephalad from the wall of the cloaca, is here seen as a three-pointed body of considerable size, projecting ventrally from the body.

Figure 8 shows in outline the enteron, from the ventral aspect, of an embryo of 20 cm. total length, or at about the time of hatching. The drawing was made from a dissection and, for the sake of simplicity, only the enteron, respiratory organs, heart, and thymus are shown. The jaw is cut through on the left side and is turned over to the right, thus bringing into view the roof of the mouth, _m_, and the dorsal side of the tongue, _tn_. At the same time the pharynx, _ph_, and the wide anterior end of the oesophagus, _oe_, are cut open, exposing the glottis, _gs_, and vocal cords, _vc_.

The lungs, _lu_, and trachea, _ta_, which are now fully formed, are dissected loose and drawn over to the right side of the animal, together with the heart, _ht_, and the thymus, _ty_; only one side of the thymus is shown, the other half being hidden by the trachea.

The mouth has reached nearly the outline of the adult. The lips are formed and, in the anterior part of the lower jaw, four tooth rudiments, _to_, are externally visible. The mucous membrane of the roof of the mouth, _m_, is covered with rounded papillae, easily seen with a lens but not shown in the figure. The tongue, _tn_, is fully formed, and is free anteriorly and laterally to about the extent that is seen in the adult; the papillae with which it is covered are not so prominent as those seen on the roof of the mouth. At the base of the tongue is the prominent transverse fold, noted in connection with figure 7, that meets above the velum palitinum, not shown here but shown in figure 7. Caudad to these folds is seen the glottis, _gs_, a triangular opening with the vocal cords, _vc_, at its base.

The mucosa of the inside of the pharynx and the anterior end of the oesophagus, exposed by the dissection, is thrown into numerous longitudinal folds, not shown in the figure; these well-marked folds extend throughout the length of the oesophagus.

The oesophagus, _oe_, tapers gradually from the wide pharynx, _ph_, and then continues as a cylindrical tube of uniform diameter to the right side of the anterior end of the stomach, where it opens into the latter organ. Its walls are thick, and its lumen is almost obliterated by the longitudinal folds of the mucosa, mentioned above.

The stomach, _i'_, is oval in outline, though somewhat flattened laterally; it is depressed, dorso-ventrally, to a little more than half the lateral diameter. As has been said, the oesophagus enters its right anterior border; the pylorus is on the right side, 3 or 4 mm. caudad to the oesophageal opening. The wall of the stomach is comparatively thin except in the region of the oesophageal and pyloric apertures, and at a point, opposite these apertures, on the left side. At the latter point is an oval or disc-shaped area that is several times as thick as the surrounding wall; it probably represents the gizzard structure of the adult. The thickening mentioned in the region of the two apertures seems to be mainly due to a wrinkling of the mucosa which, in other parts of the stomach, is nearly smooth, so far as can be seen with the naked eye.

A sphincter thickening around the oesophageal and, to some extent, around the pyloric aperture, causes each of these structures to project into the stomach like an ileo-caecal valve.

The pylorus, _py_, opens into a small, pointed, thin-walled diverticulum, _di_, and, at the same time, into the duodenum, _d_. The diverticulum noted, also, in connection with figure 7, has relatively thick, wrinkled walls; its significance is not known to the writer. From this diverticulum the duodenum, _d_, leads caudad and laterad for a short distance as a narrow tube, then suddenly expands into the widest part of the entire intestine. Into this wide part of the duodenum, 3 or 4 mm. from the pylorus, opens the bile duct, _bd_. The bile sac, _bs_, is an elongated oval body with thin walls, lying to the right of the pylorus, its connection with the liver was not seen.

Lying between the anterior end of the duodenum and the posterior end of the stomach, and extending caudad for 10 to 15 mm., in the median plane of the animal is the pancreas, _pan_. It is a long narrow body of a whitish color; its duct or ducts could not be determined by dissection.

The duodenum extends caudad, with gradually diminishing caliber, from the enlarged region mentioned above. About 10 to 15 mm. caudad to the stomach it makes a sort of double loop to the right, a wide loop, _lp_, and a close one, _lp'_, nearer the median plane. From the latter loop the intestine extends straight to the left, for a distance of about 10 mm., where it makes a small loop cephalad, _lp^2_, and then opens to the yolk-sac, _y_. The yolk-sac is shown here simply as an irregular piece of tissue, the yolk having been removed.

The anterior intestinal portal, _aip_, and posterior intestinal portal, _pip_, are in close proximity with each other.

From the posterior intestinal portal the intestine extends straight cephalad to the posterior end of the stomach, dorsal to which it forms a double loop, a wider one, _lp^3_, and a narrow one, _lp^4_. From the latter loop, _lp^4_, the intestine extends straight caudad, parallel and near to the straight region leading from the posterior intestinal portal, until it reaches the region of the loop _lp^2_, dorsal to which it forms a small loop, _lp^5_. From loop _lp^5_ the intestine, which is here of very small caliber, extends caudad for about 10 mm., where it forms another indistinctly double loop, _lp^6_.

From loop _lp^6_ the large intestine, _il_, extends, with gradually increasing caliber, to the cloaca, _cl_, a distance of 10 to 15 mm.

Except in the enlarged region near the pylorus the lumen of the intestine is almost obliterated by the folding of its thick walls, so that little or nothing can be told of its lining with the naked eye.

A distinct mesentery holds the loops of the intestine in position and binds the entire enteron close to the dorsal body wall. Because of the lack of properly fixed tissue no sections of the enteron of this stage were made.

REFERENCES

1. BRONN, H. G.: Kla.s.sen des Thier-Reichs. (Vols. on reptiles.) 1890.

2. CHAFFANJON, V.: Observations sur Alligator mississippiensis (Tractus intestinalis und Mesenterium). Ann. Soc. Linn. Lyon, vol. 28, p. 83 ff., 1881.

3. EISLER, P.: Zur Kentniss der Histologie des Alligatormagens.

Archiv f. Mikr. Anat., vol. 34, pp. 1-10, 1889.

4. HERTWIG, O.: Comparative Embryology of Vertebrates. Especially vol. 2, pp. 1-241, 1906.

5. REESE, A. M.: The Nasal Pa.s.sages of the Florida Alligator. Proc.

Acad. Nat. Sc. Phila., 1901.

6. REESE, A. M.: The Development of the American Alligator. Smith. Misc.

Coll., vol. 51, No. 1791, pp. 1-66, 1908.

DESCRIPTION OF FIGURES 1-8, PLATES 1-15

[Transcriber's Note: The word "Plate" refers to the physical pages on which the Figures were printed. The word is not used as an ill.u.s.tration identifier.

Based on the author's age at time of publication, "Miss C. M. Reese"

is more likely to have been his sister than his daughter.]

The surface views were drawn, under the author's direction, by Miss C. M. Reese. The first two of these views were copied, by permission, from S. F. Clarke; the others were drawn from the specimens themselves.

All of the figures of any one stage are given the same number, followed by distinguishing letters, so that it is possible to tell at a glance what figures belong together.

All of the figures except those from Clarke were drawn under a camera lucida.

Figure 1. A surface view of an embryo, from the dorsal aspect, at the beginning of the formation of the enteron.

Figure 1A. A sagittal section of an embryo of approximately the age of the one shown in figure 1. 43.

Figure 2. A dorsal view of an embryo with five pairs of mesoblastic somites.

Figure 2A. A sagittal section of an embryo of the stage shown in figure 2. 43.

Figure 2B. A transverse section through the headfold of an embryo of the stage shown in figure 2. 43.

Figure 3. A dorsal view of an embryo with about fifteen pairs of somites. 20.