The alligator and its allies

by A. M. Reese

About seventeen pairs of somites are present.

The amnion extends over the anterior two thirds of the embryo.

The above-mentioned increase in the diameter of this embryo over that of the preceding is evident when the first two transverse sections of this stage are compared with the corresponding sections of the earlier stage; in the middle and posterior regions there is not very much difference in size.

[Figure 12a] passes through the region of the forebrain. This end of the embryo lies on its side, as was noted above and as may be recognized from the relative positions of the head and the overlying yolk (y). The great size of this and the following figure is due partly to the increase in size mentioned above and partly to the fact that the sections pass through the region of cranial flexure. The present figure ([12a]) represents the brain cavity as large and dumbbell-shaped, with comparatively thick walls of compactly arranged cells. The ventral end of this cavity (fb) is cut anterior to the region of the optic vesicles, while the dorsal end (mb) may perhaps be called the midbrain. In the sections that follow this one the two cavities are distinct from each other. The medullary canal, as was stated above, is now completely enclosed, except for the ventral opening of the neurenteric canal, to be presently noticed. Surrounding the brain is a considerable mass of mesoblast (mes). It is composed of the typical stellate cells. The ectoderm (ec) is made up of the same irregularly and loosely arranged cells that have been seen in earlier stages; it is of unequal thickness in different regions, the thicker parts being at the sides. The amnion (a) has the usual appearance, and in this region of course completely surrounds the embryo.

[Figure 12b] is ten sections posterior to the section just described. The width of the embryo is greater in this region, but the dorso-ventral diameter is about the same as in the more anterior section.

The overlying yolk and blastoderm are not shown in any figure of the series except the first. In this [figure] the forebrain (fb) and midbrain (mb) are widely separated instead of being connected, as in the preceding figure, where the section passed through the actual bend of the cranial flexure. The anterior and ventral part of the cranial cavity, the forebrain (fb), is nearly circular in outline. It exhibits on one side a well-marked optic vesicle (ov), which is sufficiently advanced in development to show a rudimentary optic stalk. The outer wall of the optic vesicle is in close contact with the superficial ectoderm, which shows as yet no sign of the formation of a lens vesicle. The plane of the section being probably not quite at right angles to the long axis of the embryo, the optic vesicle of one side only was cut. The wall of this part of the forebrain is of about the same thickness and appearance as in the preceding stage. The other cerebral cavity (mb) of this section is probably the hinder part of the midbrain, though it may be the anterior part of the hindbrain; there is no sharp line of demarcation between these regions of the brain. This cavity (mb) is much smaller in section than the forebrain; its walls are of about the same thickness.

Ventral to the midbrain is the anterior end of the notochord (nt), surrounded by the mesoblast. At various places throughout the mesoblast irregular open spaces may be seen; these are blood-vessels. The ectoderm (ec) and amnion (a) have about the same appearance as in the preceding figure, though the former seems somewhat thinner.

[Figure 12c] is just back of the bent-under forebrain represented in the preceding figure and in front of the main body of the heart. The plane of the section not being at right angles to the long axis of the body (as was mentioned above), the figure is not bilaterally symmetrical. The neural canal, since the section passes through the auditory vesicles, may here be called the hindbrain (hb). It has an almond-shaped cavity, surrounded by a wall of medium thickness. In close contact with the wall of the hindbrain, on each side, is the inner side of the auditory vesicle (o), which is seen as a deep, wide-mouthed pit in the superficial ectoderm. On the right side of the section the auditory pit is cut through its middle region; it is simply a thickened and condensed area of the ectoderm which has been invaginated in the usual way. Directly beneath the hindbrain is the notochord (nt), on each side of which, in the mesoblast, is the dorsal aorta (ao), or rather the continuation of the aorta into the head. Beneath these structures and extending from one side of the section to the other is the pharynx (ph); its lining wall is fused on each side with the ectoderm, but there is no actual opening to the exterior. These points of contact (g) between entoderm and ectoderm are of course the gill clefts; they are not yet visible from the outside. The roof of the pharynx is flat and comparatively thin, while the floor is thickened and depressed to form a deep, wide pit, traceable through six or eight sections. This pit may be the thyroid gland already noticed in the preceding stage. Below the main cavity of the pharynx and close to each side of the thyroid rudiment just mentioned is a large blood-vessel (tr). These two vessels when traced posteriorly are found to be continuous with the anterior end of the heart, and hence may be called the truncus. They were noticed in [Figure 11c], bv. The ectoderm surrounding the lower side of the embryo was so thin and indistinct that it could scarcely be distinguished from the mesoderm of that region. The amnion (a) is still a continuous envelope entirely surrounding the embryo.

[Figure 12d], about twenty sections posterior to [Figure 12c], is in the posterior heart region. The spinal cord (sc), as might be expected, is smaller than in the more anterior region, but is otherwise not markedly different from what was there seen. The notochord (nt) also has the same appearance as before. The enteron (ent) shows of course in this region no gill clefts; it is a small, irregular cavity with thicker walls than in the figure just described. The ventro-lateral depression is entirely distinct from the depression that was called the thyroid rudiment in the preceding figure. Dorsal to the enteron are the two dorsal aortæ (ao), now smaller and more ventral to the notochord than in the preceding figure. Ventral to the enteron is the large heart (ht), projecting below the body cavity, which is no longer enclosed. The mesodermic wall (mes′) of the heart is still comparatively thin and is separated by a considerable space from the membranous endocardium (en′). The extent and shape of the heart are shown in the surface view of this stage. On the right side of the section the body cavity extends to a point nearly opposite the middle of the spinal cord, considerably dorsal to the notochord, while on the left side the dorsal limit of the body cavity is scarcely level with the lower side of the notochord. Between the dorsal end of the body cavity and the side of the spinal cord, on the left, is a dense mass of mesoblast (s), one of the mesoblastic somites. A few sections either anterior or posterior to the one under discussion will show the condition of the two sides reversed—that is, the body cavity will extend to the greater distance on the left and will be interrupted by a mesoblastic somite on the right. It is evident, then, that the upper angle of the body cavity is extended dorsally as a series of narrow pouches between the somites. The mesoblast that lines the body cavity, the splanchnopleure (sm) and somatopleure (so), is somewhat denser than the general mass of mesoblast, so that these layers are quite distinct, the former (sm) extending around the enteron (ent) and heart (ht), and the latter (so) being carried dorsalward as the mesoblastic part of the amnion (a). The amnion may be traced through about 130 of the 200 sections into which this embryo was cut.

[Figure 12e] is nearly one fourth the length of the embryo posterior to [Figure 12d]; it is approximately in the middle region. The diameter of the embryo has been gradually decreasing until now it is very much less than in the head region. The section being behind the head-fold the entoderm (en) is nearly flat and the enteron is quite unenclosed. The canal of the spinal cord (sc) is smaller in proportion to the thickness of its walls, and the notochord (nt) is somewhat larger than in the preceding sections. In proportion to its extent, the ectoderm is very thick. Under the notochord the dorsal aortæ (ao) are seen as two large, round openings in the mesoblast. On the left side the section passes through the center of a somite and shows a small, round myocœl (myc). The mesoblastic layer of the amnion (so) is distinct throughout from the ectoblastic layer (a).