The alligator and its allies

by A. M. Reese

The most striking feature of the section is the marked projection of the Wolffian ridges, though no local enlargements of these ridges indicate the rudiments of the limbs. A large mass of Wolffian tubules (wt) is seen projecting into the upper part of the body cavity on each side; close to each of these masses is the posterior cardinal vein (pc), and between them is the large aorta (ao). The other structures are about as in the preceding section.

[Figure 16g] represents a sagittal section of the anterior half of the body of an embryo of this or possibly a slightly younger stage of development. The three regions of the brain are clearly indicated, as well as the cavity of the spinal cord (sc). The roof of the hindbrain has been made too thick in the figure; it should be represented by a mere line. A little mesoblast is to be seen at places between the roof of the brain and the superficial ectoderm. A slight invagination of the epithelium (p), between the floor of the brain and the anterior end of the notochord, probably represents the beginning of the hypophysis. No indication of the paraphysis is yet to be seen. Extending from the region of the hypophysis to the posterior end of the section is the notochord (nt); it is much vacuolated and gradually increases in thickness toward the posterior, though its outline is quite irregular; except at the extreme anterior end and at one or two other places, it lies in close contact with the floor of the neural tube. Directly under the notochord lies, in the posterior half of the figure, the large dorsal aorta (ao). The pharynx (ph), opening between the end of the forebrain and the thick mandibular fold (across which opening the amnion, a, of course extends), is a funnel-shaped space which passes out of the plane of the section toward the posterior end of the figure. Its thick endodermal lining extends to the mandibular fold on the ventral side, while on the dorsal side it gradually thins out and becomes continuous with the thin ectoderm that extends over the forebrain. Just back of the mandibular fold is the bulbus (b), and back of that is the edge of the ventricle (vn). Posterior and dorsal to the ventricle the liver (li) is seen as an irregular mass of cells, and dorsal to the liver one of the Wolffian bodies (wt) is cut through its extreme edge.

STAGE XIV

Figures 17-17g ([Plates XXIII.], [XXIV.])

Body flexure has increased until now the forebrain and tail are almost in contact ([Fig. 17]). The eye has developed somewhat; the ear vesicle, which is not shown in the figure, is small and seems to lie nearer the ventral side; the nasal pit is much larger and is crescentic in shape. The hyomandibular cleft (g′) still persists as a small crescentic slit, while the next three clefts are now represented merely by superficial grooves separated by distinct ridges, the visceral folds. No indication of a fifth cleft is seen. The maxillary process (mx) grows ventralward under the forebrain and is already longer than the mandibular arch (md).

The chief advance in development over the preceding stage, besides the formation of the maxillary process, is in the appearance of the appendages (aa and pa); they have the characteristic shape of the rudimentary vertebrate appendage, though the anterior pair seem to point in an unusual direction at this stage and to be slightly more developed than the posterior. The curious, anteriorly directed heart (ht) is, perhaps, somewhat abnormal. The umbilical stalk (u) is comparatively narrow and, like the allantois, was cut off close to the body.

Transverse sections of an embryo of this stage are represented in [Figures 17a-g], drawn under a lower magnification than were any of the preceding figures.

[Figure 17a] is in the region of the pharynx, and passes through the forebrain (fb) and posterior part of the hindbrain (hb). In the thick walls of both of these structures histological differentiation has begun, so that even under low power an inner granular and an outer clear zone may be distinguished. Under greater magnification the presence of short fibers may be made out among the cells. The cerebral hemispheres (ch) are well-marked structures, their asymmetry being of course due to the obliquity of the section. Only one eye is cut by the plane of the section, and this one shows no connection with the forebrain. The outer wall of the optic cup (oc) is so thin that under this magnification it can scarcely be seen as a dark line surrounding the retinal wall. The lens (ln) is now a solid mass, of the usual type for vertebrate embryos, its front or outer wall being a scarcely discernible line. The hindbrain (hb) has the usual form for that region and does not differ particularly from what was noted in earlier stages except in the histological differentiation that has already been mentioned. As with the eye, it is only on the right side that the auditory vesicle (o) is shown. It shows some differentiation, but not so much as would be seen were it cut in another region. In the center of the section the pharynx (ph) forms an irregular cavity connected with the exterior on the left by a gill cleft (g) and by another slit which is simply the anterior margin of the stomodæum. On the right neither of these openings is in the plane of the figure, though the gill cleft (hyomandibular), which lies close to the auditory vesicle, is almost an open passage. A few small blood-vessels are scattered through the section; one of these (bv), lying between the notochord (nt) and the floor of the brain, is noticeable from its being very closely packed with corpuscles, so that at first glance, under low magnification, it looks more like a nerve than a blood-vessel.

[Figure 17b] is also through the pharyngeal region, a short distance behind the preceding section. The growth of the cerebral hemispheres (ch) is better shown than in the preceding figure, as is also the general form of the optic cup (oc). On the left the nasal cavity (n) is seen as an elongated slit with thick walls; it is cut near, but not through, its opening to the exterior. The same gill cleft (g) that was seen in the preceding figure is seen here as a narrow, transverse cleft, open at both ends. Between the notochord (nt) and the spinal cord (sc) is the same, though now double, blood-filled vessel (bv) that was seen in the preceding section. The other blood-vessels are larger here than in the more anterior region. There is a faint condensation of mesoblast in the neighborhood of the notochord, and a more marked condensation (mp) farther toward each side is the curiously shaped muscle plate.

[Figure 17c] is through the heart region, and that organ is cut through the opening from the lower or ventricular into the upper or auricular chamber. The thickening of the wall of the ventricle, which was noticed in the preceding stage, has increased to such an extent that there is now a marked difference in the thickness of the ventricular and auricular walls. As in the preceding stage, the body wall is torn, probably in handling, so that it appears to be incomplete around the ventral side of the heart. Dorsal to the heart two small circular holes (ent) with thick walls are the œsophagus and trachea, cut anterior to the point of bifurcation of the latter into the bronchial or lung rudiments. On either side of these structures is an elongated blood-vessel (ac), the anterior cardinal vein, its elongation being due to the fact that it is cut at the place where it turns downward to empty into the heart. Dorsal to the œsophagus are the aortæ (ao), which are here cut just at the point where the two vessels unite to form one; the next section, posterior to the one under discussion, shows an unpaired aorta. The notochord (nt) and spinal cord (sc) need no description, except to note that the latter shows active histological differentiation, numerous mitotic figures being seen under higher magnification, especially in the cells that line the spinal canal. On the right of the cord the edge of a spinal ganglion (sg) is seen, in connection with which in other sections are seen the clearly defined nerve roots. The condensation of mesoblast around the notochord is quite evident, and in close contact with this medial condensation are two very characteristic, S-shaped muscle plates (mp), which extend from the level of the dorsal side of the spinal cord to the upper limits of the cardinal veins. In some sections the muscle plates even yet show slight remains of the myocœl at the dorsal end.