The alligator and its allies

by A. M. Reese

The embryo represented by this series of transverse sections is intermediate in development between those represented in surface views by [Figures 8] and [10]. The amnion and head-fold are nearly the same as in [Figure 8]; the medullary folds are intermediate in development, the anterior end not showing so marked an enlargement as shown in [Figure 10], v′. There are six or seven faintly distinguishable somites.

[Figure 9a] represents a section through the anterior part of the head-fold; it shows one unusual condition: the head lies entirely beneath the surface of the yolk. This condition is quite confusing when the section is studied for the first time. The pushing of the head under the yolk is shown at its commencement in [Figure 11]. The process continues until nearly the entire anterior half of the embryo is covered; but when the embryo attains a considerable size it is seen to lie entirely above the yolk, as in the chick. According to Voeltzkow’s figures ([78]) this same condition is found in the crocodile, and Balfour also mentions it in connection with the lizard. The fusion of the medullary folds has made considerable progress, so that the entire anterior end of the canal is enclosed, except in the region where the folds are bent down and back, as in the preceding stage; here the folds are still distinct from each other, leaving the medullary canal open on the ventral side, as shown in [Figures 9a and 9b]. In the section under discussion the ectoderm (ec) is a very thin membrane on top of a considerable mass of yolk, while no entoderm can be distinguished. The amnion (a) completely surrounds the embryo as an irregular membrane of some thickness in which no arrangement into layers can be seen. The epidermal ectoderm (ep) is composed of the usual loosely arranged cells, so that it is clearly distinguishable from the compactly arranged cells of the nervous layer (nl), from which it is separated by only a line.

In [Figure 9b], which shows a section a short distance posterior to the preceding, the medullary canal (mc) is somewhat deeper and is still open ventrally. There is a distinct space between the nervous (nl) and epidermal (ep) layers of the ectoderm, in which space a few mesoblast cells (mes) may be seen. The section is cut just posterior to the edge of the amnion, so that there is now neither amnion nor yolk above the embryo.

[Figure 9c] is about ten sections posterior to [Figure 9b]. The section passes through the anterior wall of the bent-under part of the medullary canal (mc′), so that the actual canal is shown only on the dorsal side (mc), where it is completely closed and begins to assume the shape of the typical embryonic spinal cord. The space between the superficial (ep) and nervous (nl) layers of the ectoderm is quite extensive and is largely filled by a fairly compact mass of mesoderm (mes).

[Figure 9d], although only five sections posterior to the preceding, shows a marked change in structure. The medullary canal (mc) is here of the typical outline for embryos of this age. A large, compact mass of cells (ent) appears at first glance to be the same that was noted in the preceding stage at the tip end of the turned-under medullary canal; it is, however, the extreme anterior wall of the enteron, which is in close contact with the above-mentioned tip of the medullary canal. Between this anterior wall of the enteron, of which wall it is really a part, and the medullary canal is the notochord (nt). The space surrounding the notochord and enteron is filled with a fairly compact mass of typical, stellate mesoblast cells. The depression of the ectoderm (ec) and entoderm (en) of the blastoderm caused by the formation of the head-fold is here less marked, and the dorsal side of the embryo in this region is slightly elevated above the level of the blastoderm.

[Figure 9e] represents a section passing through the posterior edge of the head-fold. The epidermal ectoderm is here continuous with the thin layer of superficial ectoderm (ec) of the blastoderm, while the entoderm (en) of the blastoderm is still continuous beneath the embryo. The thick ectoderm of the embryo is sharply differentiated from the thin layer of ectoderm that extends laterally over the yolk. The pharynx (ent) is a large cavity whose wall is thick except at the dorsal side, where it is thin and somewhat depressed, apparently to make room between it and the medullary canal for the notochord (nt).

[Figure 9f] is about twenty sections posterior to the preceding section, and passes through the point of separation of the folds of the entoderm (en). From this point the entoderm gradually flattens out, leaving the enteron unenclosed. The medullary canal (mc) and notochord (nt) are about as in the preceding section, but the ectoderm (ep) is somewhat thinner and more flattened. The mesoderm (mes) on the right side exhibits a distinct cleavage, the resulting body cavity (bc) being a large, triangular space.

[Figure 9g], the twenty-fifth section posterior to that represented in [Figure 9f], shows a marked change in the form of the embryo. While of about the same lateral dimensions, the dorso-ventral diameter of the embryo in this region is less than one half what it was in the head region. The epidermal ectoderm (ep) is now nearly horizontal in position and is not so abruptly separated laterally from the thin lateral sheets of ectoblast. The medullary groove (mg) is here a very narrow vertical slit. At this stage the fusion of the medullary folds has taken place over the anterior third of the embryo. For a short distance, represented in about thirty-five sections, the canal is open as in the figure under discussion; for the next one hundred sections (about one third the length of the embryo) in the region of the mesoblastic somites the canal is again closed, while throughout the last one third of its length the canal is widely open dorsally. The enteron is here entirely open ventrally, the entoderm being almost flat and horizontal. The notochord (nt) is distinctly outlined and is somewhat flattened in a dorso-ventral direction. The body cavity (bc) is well marked, but is separated by a considerable mass of uncleft mesoblast from the notochord and the walls of the medullary groove.

A space of about one hundred sections, or one third the length of the embryo, intervenes between [Figures 9g] and [9i]. This is the region of the mesoblastic somites, and in this region, as has been above stated, the medullary canal is completely enclosed. It is evident then that the entire anterior two thirds of the medullary canal is enclosed except for the short region represented in [Figure 8g]. Whether or not this short open region between the two longer enclosed regions is a normal condition the material at hand does not show.

[Figure 9h] represents a typical section in the region of the mesoblastic somites just described. It shows the enclosed medullary canal (mc), the body cavity (bc) on the right, and a mesoblastic somite with its small cavity (myc) on the left. The entire section is smaller than the sections anterior or posterior to this region, and seems to be compressed in a dorso-ventral direction, this compression being especially marked in the case of the notochord.