The alligator and its allies

by A. M. Reese

[Figure 17d] is in the region of the posterior end of the heart (ht), which is cut through the tip of the ventricle, and the anterior end of the liver (li), which has the appearance of a mass of darkly stained cords or strands of cells surrounding a large blood-vessel (mv). This blood-vessel may be called the meatus venosus, though it is not separated by any line of demarcation from the auricle. A few sections anterior to this region the meatus venosus opens dorsally into a large vessel on each side (dc), which at first glance seems a part of the body cavity, but which is in reality the ductus Cuvieri, formed by the union of the anterior and posterior cardinal veins. An irregular, crescentic cleft (bc), lying medial and parallel to each of the Cuvierian vessels, is the body cavity. In the upper angle of this cavity is a granular mass, the glomerulus, that of the left side being accompanied by the extreme anterior end of the Wolffian duct. In the rounded mass of mesoblast, between the cleft-like regions of the body cavity, the lung rudiments (lu), and the œsophagus (oe) are seen as three small, circular openings; that of the œsophagus is somewhat smaller than the other two. The notochord (nt), spinal cord (sc), and muscle plates (mp) have almost the same appearance as in the preceding section. A spinal ganglion (sg) is seen on each side of the spinal cord; the one on the left shows a well-defined spinal nerve (sn), which may be traced ventrally as far as the end of the muscle plate, along whose medial side it courses. The ventral nerve root is plainly seen; the dorsal root, in this section, less plainly. The amnion (a) and abdominal wall are, as in the preceding figure, torn in the region of the ventricle.

[Figure 17e] is a short distance posterior to the figure just described. The liver is cut through its middle region and forms a large, darkly staining, reticular mass on the left side of the figure. The digestive tract is seen at two places to the right of the liver; the smaller and more ventral of these openings (i) may be called the intestine, while the larger is evidently the stomach (i′). The body wall is here unfused and becomes suddenly thinner as it passes upward into the amnion (a). The Wolffian tubules (wt) form a very conspicuous mass on either side of the mesentery, in close connection with the posterior cardinal veins (pc). In the mesoblast between the dorsal aorta (ao) and the notochord are two small, irregular, darkly stained masses (sy). These are shown in the preceding two figures, but were not mentioned in the description. They may be traced through a great part of the length of the embryo back of the head region; at intervals corresponding in length to the distance between the spinal ganglia they are enlarged, while between these enlargements they are very small in cross-section. At certain points a small blood-vessel is given off by the dorsal aorta to the immediate neighborhood of each of these small areas. Although they show no connection with the central nervous system, these structures appear to be the rudiments of the sympathetic nerves.

[Figure 17f] is in the region just in front of the hind legs. The abdominal walls are here unfused, and into the unenclosed body cavity projects the intestine (i), supported by a narrow mesentery and surrounded by a comparatively thick mass of mesoblast. The Wolffian body and duct form a mass of considerable size on each side of the mesentery. The Wolffian body is cut near its posterior end and consists of smaller tubules than in the more anterior regions. The Wolffian ducts (wd), on the other hand, are very large and are much more clearly distinguishable from the Wolffian tubules than in the preceding sections. The Wolffian ridges (wr) are very marked projections on the sides of the body, and in a region farther caudad become especially developed as the posterior appendages, to be described in connection with the following section. Both spinal ganglia are shown in this figure (sg), and in connection with the left ganglion the spinal nerve (sn), extending ventrally as far as the level of the Wolffian duct. The sympathetic nerve rudiments do not extend so far caudad as the plane of this section. The dorsal end of each muscle plate (mp) is seen, in this and other sections, to be slightly enlarged to form a round knob; this knob contains a distinct cavity (not shown in the figure), the myocœl.

In [Figure 17g], owing to the curvature of the body, the plane of the section passes through the body at three places: through the region of the heart and lungs ([Fig. 17d]), through the region of the posterior appendages, and through the tail. In fact, the plane of the section represented by each of the preceding figures cut the embryo in more than one region, but for the sake of simplicity only one region was represented in each figure. In the figure under discussion only the leg and tail regions have been drawn, though the latter region (t), being cut through one of its curves, is seen as an elongated body with a section of the spinal cord, notochord, etc., at each end. Both regions shown in the figure are enclosed in the same fold (a) of the amnion. Of the structures in the dorsal side of the larger or more anterior part of this figure nothing need be said. The most striking feature of the section is the presence of the large posterior leg rudiments (pa). As was noted in the preceding figure, they are, as usual, merely local enlargements or projections of the mesoblast (covered, of course, with ectoblast) of the Wolffian ridge. They are, as shown in this section and in the surface view of this stage ([Fig. 17]), bluntly pointed projections from the sides of the body. The anterior appendage seems to be slightly more developed than the posterior, as was noted in describing the surface view of the embryo. The digestive tract is cut through its extreme posterior end, in the region that may be termed the cloaca (cl), for into it at this point the Wolffian ducts open (wdo). As the narrow cloacal chamber is followed toward the tail, it becomes still smaller in diameter, and the ventral depression or cleft seen in this figure gradually becomes deeper until its walls are continuous with the ectoderm that covers the ventral projection of mesoderm between the hind legs; no actual opening to the exterior is present, however. There is a space of about twenty-five or thirty sections (in a series of eight hundred) between the posterior ends of the Wolffian bodies and the cloacal openings of the Wolffian ducts. The body cavity (bc) and the posterior cardinal veins (pc) are very small in this region, as might be expected.

STAGE XV

Figure 18 ([Plate XXIV.])

Only the head of this embryo is represented, as the general state of development is about the same as in the preceding stage.

The chief object in making the figure is to show the five gill clefts (g¹⁻⁵). The fifth cleft, though small and probably not open to the exterior, is quite distinct in this embryo. The writer would feel more doubt of its being a true, though rudimentary, gill cleft had not Clarke ([17]) found a fifth pair of clefts in his material. The nasal pit has advanced in development somewhat and shows the beginning of the groove that connects it with the mouth. In this figure the crescentic hyomandibular cleft shows its connection with the groove between the mandibular and the hyoid folds.

STAGE XVI

Figure 19 ([Plate XXIV.])