The alligator and its allies

by A. M. Reese

The head-fold is not so striking a feature as in the preceding stage. The head-fold of the amnion (a) now covers nearly two thirds of the embryo. The heart (ht) is seen as a dark, rounded object projecting to the right side of the neural canal, just anterior to the first somite. The vitelline blood-vessels are just beginning to form, but are not shown in the figure.

The depression of the anterior region that was noted in the preceding stage has advanced so far that a considerable part of the embryo now projects forward under the blastoderm. In some cases it is almost concealed in a dorsal view; in other cases it may easily be seen through the transparent membranes, especially after clearing.

In opening eggs of this stage one is at first apt to underestimate the size of the embryos, since the anterior part of the embryos cannot be seen until after they are removed from the yolk and are viewed from the ventral side.

The embryo from which the series of transverse sections of this stage was made, while of the same state of development as that shown in [Figure 11], was more fully covered by the blastoderm than is shown in the surface view in question.

[Figure 11a] passes through the tip of the head. Dorsal to the embryo is the ectoderm and a thick mass of yolk (y). The amnion (a) is seen as an irregular membrane which entirely surrounds the head. The medullary canal (mc) is entirely closed except at the extreme anterior end, which is bent downward so that the opening is on the ventral side. The nervous (nl) and epidermal (ep) layers of the ectoderm are in contact throughout, but are clearly distinguishable because of the difference in the compactness of their cells.

In [Figure 11b] is represented a section, behind the preceding, which passes through the posterior tip of the turned-under anterior end (mc′). Here the medullary canal is closed both above (mc) and below (mc′). The amnion (a) has about the same appearance as in the more anterior section, but there is here a considerable space, filled with mesoblast (mes), between the nervous (nl) and epidermal (ep) layers of ectoderm.

[Figure 11c] is twenty sections, about one tenth the length of the embryo, posterior to the one last described. The large mass of overhanging yolk (y) is still present, as is also the amnion (a), though the latter no longer passes entirely around the embryo; only the true amnion could be made out. The thickened walls of the medullary canal have reduced that cavity to a narrow, Y-shaped slit (mc). The notochord (nt) is very slender in this region, compared to its diameter farther toward the posterior end. The enteron (ent) is a large cavity, whose wall is made up of loosely arranged cells except around a median, ventral depression where the cells are more compact. This depression may be traced through ten or fifteen sections and may represent the beginning of the thyroid gland, though this point was not worked out with certainty. Surrounding the notochord and enteron is a loose mass of typical, stellate mesoblast cells (mes), which are cleft on either side to form the anterior limit of the body cavity (bc). Between the body cavity below and the enteron above, on each side, is a small blood-vessel (bv) which when followed caudad is found to open ventrally and medially into the anterior end of the heart.

[Figure 11d] is about a dozen sections posterior to the preceding. The appearance of the overhanging yolk (y), of the amnion (a), and of the notochord (nt) is about as in the more anterior section. The medullary canal (mc) is a straight, vertical slit, and the depression in the floor of the pharynx (ent) is much more shallow. The body cavity (bc) is much larger and extends across the mid-ventral line beneath the heart (ht), which is cut through its middle region. The heart may be traced through about twenty sections (one tenth the length of the embryo); its mesoblastic wall (mes′) is thin and irregular, and is lined by a distinct endothelium (en′) whose exact origin has not yet been worked out.

[Figure 11e] is just back of the heart, and shows in its place the two vitelline veins (vv). The depression in the floor of the enteron (ent) is entirely distinct from the one that has been mentioned above, and is simply the posterior limit of the head-fold of the entoderm; the fifth section posterior to this shows where this depression opens ventrally to the yolk sac. The other structures shown in the figure are not markedly different from what was seen in [Figure 11d].

[Figure 11f] is about one tenth the length of the embryo posterior to [Figure 11e]. The chief differences here noticed are in the enteric and cœlomic cavities. The former is no longer enclosed, a dorsal fold in the entoderm being all that remains of the cavity that was seen in the more anterior figures, while the latter is here reduced to a narrow cleft between the somatic and splanchnic mesoblast. A thickening of the mesoblast on either side of the notochord, especially on the left, represents a mesoblastic somite. The medullary canal (mc) is more open than in the more anterior sections.