The alligator and its allies

by A. M. Reese

[Figure 20]e is only a short distance posterior to the preceding. On the left side the pharynx (ph) is connected with the exterior through the stomodæum, and on the right the hyomandibular cleft (g′) is cut almost through its opening to the exterior. The auditory vesicle (o) on the right is cut near its middle region, while that on the left is barely touched by the plane of the section. The notochord (nt), with its condensed area of mesoblast, is somewhat larger than in the preceding section. The nasal canal on the right (n) is cut through neither anterior nor posterior opening, while on the left side the canal shows the anterior opening (an).

[Figure 20f], which is in the region of the posterior part of the pharynx and the anterior part of the heart, shows some rather unusual conditions.

The spinal cord (sc) and notochord (nt), with the faintly outlined condensations of mesoblast in their region, need no special description. The pharynx (ph) is here reduced to an irregular, transversely elongated cavity, the lateral angles of which are connected on each side with the exterior through a tortuous and almost closed gill cleft (g), which must be followed through many sections before its inner and outer openings may be determined. Dorsal to the pharynx numerous blood-vessels (bv), both large and small, may be seen, while ventral to it is noticed a faint condensation of mesoblast (la), in the form of an inverted T, the anlage of the laryngeal structures. The ventral portion of the figure is made up of a nearly circular thin-walled cavity, the pericardium (pr). Most of the pericardial cavity is occupied in this section by the thick-walled ventricle (vn), above which is the bulbus (b) and the tip of the auricle (au). The bulbus is nearly circular in outline, though its cavity is very irregular. A few sections anterior to this, the opening of the bulbus into the ventricle is seen.

In [Figure 20g] the section represented is only a short distance posterior to the one represented by [Figure 20f]. The mesoblastic structures in the neighborhood of the spinal cord (sc) and notochord (nt) will be described in connection with the next figure, where they are more clearly defined. The œsophagus (oe)—or posterior end of the pharynx, whichever it may be called—is here a crescentic slit, with its convex side upward; ventrally it opens by a narrow glottis into the trachea (ta). The trachea is surrounded by the same condensed area of mesoblast (la) that was mentioned in connection with the preceding figure, but the condensation is here more marked. From the bulbus (b) an aortic arch (ar) extends upward for a short distance on the right side, while to the left of the œsophagus an aortic arch (ar) is cut through the upper part of its course. Ventral to the bulbus the ventricle (vn) and two auricles (au) are seen surrounded by the pericardial wall.

[Figure 20h] is in the region of the liver (li), which has about the same position in relation to the auricles (au) that was occupied by the ventricle in the last figure. The auricles are connected with each other by a wide passage. The trachea (ta) and the œsophagus (oe) are entirely distinct from each other; the former is a small, nearly circular hole, while the lumen of the latter is obliterated and its walls form a solid, bow-shaped mass of cells. Since there is a narrow space between this mass of cells and the surrounding mesoblast, it might be thought that the lumen of the œsophagus had been closed by the simple shrinkage of its walls; higher magnification, however, fails to show any sign of a collapsed lumen. It is doubtless the problematic and temporary closure of the œsophagus that is noticed in other forms. On each side of the œsophagus, in close relation with the anterior cardinal vein (ac), is noticed a nerve (cn) cut through a ganglionic enlargement. When traced forward these nerves are seen to arise from the region of the medulla, and when followed caudad they are found to be distributed chiefly to the tissues surrounding the newly formed bronchi; they are doubtless the tenth cranial nerves. On the right side of the figure the close connection of this nerve with the near-by gill cleft is seen. Above the paired aortæ (ao) the sympathetic nerves (sy) will be noticed. The mesoblast surrounding the spinal cord (sc) and notochord (nt) is distinctly condensed (more so than the figure shows) to form what may be called the centrum (c) and neural arch (na) of the vertebræ. The arch, owing to the slight obliquity of the section, shows here only on one side. The spinal cord is not yet completely enclosed by the neural arches. The muscle plates (mp) are in close connection with the rudiments of the vertebræ just mentioned. The spinal cord (sc) is here differentiated into three areas—a dense, deeply stained area immediately around the neurocœl; a less dense area of cells surrounding the inner area and extending ventralward as a rounded projection on each side; and an outer layer, with few or on nuclei, surrounding the inner two layers except on the dorsal side.

In [Figure 20i] the size and complexity of the figure are due, it will be easily understood, to the fact that the plane of the section passed through the curve of the body, thus practically cutting the embryo in two regions—an anterior, where the lungs (lu) and liver (li) are seen, and a posterior, where the Wolffian bodies (wt) are present. The spinal cord and the surrounding structures have almost the same characteristics at both ends of the figure, except that the primitive spinal column is rather more distinct in the posterior end of the section. The posterior cardinal veins (pc), Wolffian ducts (wd), and Wolffian bodies (wt) are also prominent structures of this end of the figure, the last being made up of a great number of tubules. The extreme anterior ends of the Wolffian bodies are seen in the other half of the section in the upper angles of the body cavity, dorsal to the lung rudiments (lu). Filling most of the body cavity (bc) and making up the greater part of the middle of the figure are the liver (li), now a very large organ; the stomach (i′), also quite large; the pancreas (pan), a small body lying near the stomach; and the lungs (lu), which here consist of several thick-walled tubes, surrounded by lobes of mesoblast. The other features of the figure need no special mention.

[Figure 20j] is through the base of the posterior appendages (pa), in which the cartilages are already being outlined by condensations of mesoblast. The intestine (i) is cut in two regions—at a more anterior point, where it is seen as a small, circular hole surrounded by mesoblast and hung by a narrow mesentery, and through the cloacal region, the larger and more ventral cavity, into which the Wolffian ducts (wd) open a short distance caudad to this section. The blood-vessels present a rather curious appearance. A short distance anterior to this point the aorta has divided into three, or it might be said that it has given off two, large branches. These two branches, one on either side near the posterior cardinal vein, pass toward the ventral side of the embryo on each side of the cloaca and end at about the region represented by the present figure. The small portion of the aorta that remains after the giving off of the two branches just described continues, as the caudal artery (ca), into the tail; it is a small vessel just under the notochord, and gives off small, paired branches at regular intervals toward the vertebral region. The posterior cardinal veins (pc), posterior to the openings of the Wolffian ducts into the cloaca, unite to form a large caudal vein lying just ventral to the caudal artery.

STAGE XVIII

Figure 21 ([Plate XXVII.])

This embryo, as may be seen, for example, by the form of the appendages, is slightly further developed than the one represented in [Figure 20]. The figure is from a photograph of a living embryo as it lay in the egg, a portion of the shell and shell membranes having been removed. The embryo, which lies on its left side, is rather faintly outlined because of the overlying allantois. The allantois has been increasing rapidly in size, and is here so large that it extends beneath the cut edges of the shell at all points except in the region in front of the head of the embryo, where its border may be seen. Its blood-vessels, especially the one that crosses the head just back of the eye, are clearly shown in the figure, and in the living specimen, when filled with the bright red blood, they form a most beautiful demonstration. As in the chick, the allantois lies close beneath the shell membranes and is easily torn in removing them.