The alligator and its allies

by A. M. Reese

The most important structures to be here noted are the first rudiments of the Wolffian ducts (wd). They are seen in the present section as lateral ridges of mesoblast projecting outward and upward toward the ectoblast, which suddenly becomes thin as it passes over them. These ridges or cords of mesoblast are as yet quite solid. They arise suddenly at about the eightieth section of the series of two hundred and may be traced through about forty sections, or one fifth of the length of the embryo. Their exact length is difficult to determine because, while their anterior ends are blunt and sharply defined, they taper so gradually posteriorly that it is hard to tell just where they end. They apparently originate anteriorly and gradually extend toward the tail. In a slightly younger embryo the rudimentary Wolffian duct could be seen as a still smaller rod of cells extending posteriorly for a few sections, from the seventy-fifth section of a series of about two hundred. In the particular series under discussion the left rudimentary Wolffian duct was about one fifth longer than the right one.

[Figure 12f] is just posterior to the head-fold of the amnion, passing, in fact, on the left side through the extreme edge of its lateral fold, which is shown as a upward bend in the ectoblast and somatopleure.

The ectoblast (ec) shows the same remarkable thickening that was noted in the corresponding region of the preceding stage. The spinal cord (sc), notochord (nt), aortæ (ao), and entoderm (en) need no special mention. The mesoderm seems to be separated by unusually wide spaces from both ectoderm and entoderm, and is made up of rather closely packed cells except around the aortæ, where there seems scarcely enough tissue to hold these vessels in place. The body cavity (bc) is large, and a small myocœl (myc) is seen on the left.

[Figure 12g] is through the neurenteric canal (nc), a distinct opening through the floor of the spinal canal. The section is of course just back of the posterior end of the notochord. The entoderm (en) along the margin of the neurenteric canal is naturally continuous with the wall of the spinal cord (sc). The ectoderm (ec) is thicker than ever, except in the median plane, where it passes over the spinal cord. The mesoblast is more abundant than in the preceding figure, and shows on the left what appears to be a distinct myocœl (myc), though in surface view the mesoblastic somites do not extend this far toward the tail.

STAGE X

Figures 13-13g ([Plates XIX.], [XX.], [XXI.])

This embryo ([Fig. 13]) is about 5 mm. in length, and hence is slightly smaller than the preceding stage, though somewhat more advanced in development. The medullary canal is still apparently unclosed for a short distance at the extreme posterior end; this appearance is probably due to the neurenteric canal (nc) and to the thinness of the roof of the medullary canal rather than to any lack of fusion of the medullary folds. The optic vesicle is more distinct than in the preceding stage; a somewhat similar, though smaller, opacity (o) marks the position of the ear. There are now about twenty pairs of somites, though it is difficult to determine their exact number on account of the torsion of the body. The amnion is at about the same stage of development as in Stage IX. The heart (ht) is a large double mass, whose outlines may be dimly seen when the embryo is viewed by transmitted light. The vitelline vessels (vv) are still but faintly outlined in the vascular area; the veins and arteries cannot yet be distinguished from each other. The gill clefts, though not visible externally in the embryo drawn, may be seen in sections of this stage as evaginations of the wall of the pharynx.

The transverse sections of this stage are slightly more advanced in development than was the embryo that has just been described in surface view. Only those sections have been figured which show a decided advance in the development of some special structures over their condition in the preceding stage. The sections of the preceding stages were drawn under a magnification of eighty-seven diameters; those of this and the following stage were drawn under a magnification of only forty-one diameters. All of the figures have been reduced one half in reproduction.

[Figure 13a] is the most anterior section of this series to be described. On account of the cranial flexure, which causes the long axis of the forebrain to lie at right angles to that of the spinal cord, this section cuts the head region longitudinally. The ectoderm (ec) is of varying thickness, the thickest areas being on each side of the forebrain; it is more compact than in the earlier stages, and, owing to the low magnification under which it is drawn, it is represented here by a single heavy line. Under this magnification only the nuclei of the mesoderm cells (mes) can be seen, so that this tissue is best represented by dots, more closely set in some places than in others. The forebrain is an elongated cavity (fb) with thick, dense walls. Attached to each side of the forebrain is an optic vesicle (ov), which is considerably larger than in the preceding stage. The connection between the cavity of the forebrain and that of the optic vesicle is not seen in this section; it is a wide passage that may be seen in several sections posterior to the one under discussion. The beginning of the invagination of the optic vesicle to form the optic cup may be seen on both sides, but more plainly on the right. On the right side also is noticed a marked thickening of the ectoderm, which is invaginated to form a small pit, the lens vesicle (lv); on the left side the section is just behind the lens vesicle. Above the optic stalk on each side, in the angle between the optic vesicle and the side of the forebrain, is a small blood-vessel (bv). Several other blood-vessels may be seen at various places in the mesoblast, four of them near the pharynx being especially noticeable. The hindbrain (hb) is wider than, but not so deep as, the forebrain; its walls are very thick laterally, but are thin on the dorsal and ventral sides. The dorsal wall is reduced to a mere membrane, which, with the overlying ectoderm, has been pushed into the brain cavity, as is generally the case with such embryos. Close to the ventral wall of the hindbrain the notochord (nt) is seen. The character of the notochord has already begun to change; the cells are becoming rounded and vacuolated, with but few visible nuclei except around the periphery of the notochord. Near the center of the section, close to the ventral end of the forebrain, is the pharynx (ph), cut near its anterior limit; it is here a small, irregularly rectangular cavity with a comparatively thin wall. On the left side of the pharynx the first gill cleft (g) is indicated as a narrow diverticulum reaching toward the ectoderm. A few sections posterior to this one the first gill cleft is widely open to the exterior. As has been said, in the surface view of this stage above described none of the gill clefts showed; so that in this respect at least the sectioned embryo was more nearly of the state of development of the embryo represented in [Figure 14], to be described later.

[Figure 13b], about forty sections posterior to [Figure 13a], passes through the hindbrain in the region of the ears. Being back of the region affected by cranial flexure, this section is of course of much less area than the preceding. The ectoderm shows no unusual features; it is of uniform thickness except where it becomes continuous with the entoderm around the mandibular folds (md); there it is somewhat thickened. The most striking feature of the section is the presence of two large auditory vesicles (o). The section being not quite at right angles to this part of the embryo, the vesicles are not cut in exactly the same plane; the one on the left is cut through its opening to the exterior, while the one on the right appears as a completely enclosed cavity. In a section a short distance posterior to this one the appearance of the vesicles would be the reverse of what it is here. As may be seen in the figure, the vesicles are large, thick-walled cavities lying close to the lateral walls of the hindbrain. The hindbrain itself has the usual triangular cross-section, with thick lateral walls and a thin, wrinkled dorsal wall. Close to the ventral side of the hindbrain lies the notochord (nt), on each side of which, in the angle between the brain and the auditory vesicles, is a small blood-vessel (bv). Ventral to these structures and close to the dorsal wall of the pharynx (ph) are the two large dorsal aortæ (ao). The ventral side of the section passes through the open anterior end of the pharynx (ph). On the left is seen the widely open hyomandibular cleft (g′), between the main body of the section and the mandibular arch (md). On the right side the plane of the section was such that the hyomandibular cleft was not cut through its external opening. In each mandibular fold a large aortic arch (ar) is seen, and also a slight condensation of mesoblast, the latter probably being the forerunner of cartilage.