The alligator and its allies

by A. M. Reese

This embryo is only slightly more developed than the preceding. Body flexure is so great that the forebrain and tail nearly touch. Only the anterior three gill clefts are visible. The maxillary process (mx) is longer and more narrow; the mandibular fold has not changed appreciably. The nasal pit (n) is now connected by a distinct groove with the stomodæum. The appendages have increased in size, the posterior (pa) being the longer. The anterior appendage (aa) is distinctly broadened to form the manus, while no sign of the pes is to be seen at the extremity of the posterior appendage. The heart (ht) is still very prominent. The stalk of the umbilicus (u), which is quite narrow, projects from the ventral wall in the region between the heart and the hind legs. The tail is of considerable length and is closely coiled.

STAGE XVII

Figures 20-20j ([Plates XXV.], [XXVI.])

The superficial changes noted in this stage chiefly concern the head, which has increased considerably in length ([Fig. 20]). The curvature of the body is slightly more marked, and the tail is more tightly coiled at the end. There are still signs of three gill clefts. The maxillary process (mx) is long and narrow, while the mandibular arch (md) is still short and broad. The fronto-nasal region has greatly increased and has the aquiline profile noted by Clarke. The nasal groove has disappeared, and there remains the small opening (n) at the side of the fronto-nasal region, near the end of the still separate maxillary process. The umbilicus is in about the same condition as in the preceding stage, but the heart is less prominent. The outline of the manus (ma) is more definite, and the extremity of the posterior appendage is distinctly flattened out to form the rudimentary pes (pe). The position of the elbow-joint in the anterior appendage is seen at the end of the reference line aa.

Typical transverse sections of this stage are shown in [Figures 20a]-[j].

[Figure 20a] is a section through the middle region of the head, cutting the hindbrain on one side and the forebrain on the other. The walls of the brain show rather more histological differentiation than was seen in the preceding sections, though this cannot be shown under the low magnification used. The hindbrain (hb), which is cut near its anterior border, exhibits the usual membranous dorsal and thick ventral walls. The forebrain is here seen as three distinct cavities—a median third ventricle (tv), with a thick ventral wall, and a thin dorsal wall extended to form a large paraphysis (epi), and two lateral ventricles (ch), the cavities of the cerebral hemispheres, whose walls are quite thick except on the side next the third ventricle. The sections of this series being slightly oblique, the eye is here cut on the right side only, where it is seen as a large, semicircular cavity (e) with thick, dense walls. The mesoblast, in which several blood-vessels (bv) are seen, exhibits three distinct areas—a median, lighter zone, with a more dense area on either side. The significance of this variation in the density of the mesoblast is not apparent.

[Figure 20b] is only a few sections posterior to the section just described. It is drawn chiefly to show the appearance of the forebrain, the other structures being about as in the preceding figure, except that both eyes (e) are here represented. The section passes through the wide opening between the third (tv) and the lateral ventricles (ch) and cuts the anterior edge of the pineal body[8] (epi). The paraphysis is very large and is directed backward instead of forward, as is usually the case among the lower vertebrates (if the alligator may be so classed). It is shown in [Figure 17a] of a preceding stage and will be again shown in a sagittal section to be described later. The same areas of more dense and less dense mesoblast noted in the preceding figure are seen here.

[8] Subsequent investigation showed that the structure here described as the pineal body is, in reality, the paraphysis; the pineal body is absent in A. mississippiensis.

[Figure 20c], though still in the head region, shows several features that were not seen in the preceding figures. On the left of the hindbrain (hb) the auditory vesicle (o), which is now considerably more advanced than in earlier figures, is seen as a larger, flask-shaped cavity and a smaller, round one. Between the larger cavity and the hindbrain is the root of a cranial nerve (cn), apparently the eighth, since in another section it comes in close contact with the wall of the larger part of the auditory vesicle just mentioned. On the right side, ventral to the hindbrain, another and much larger nerve (cn) is seen. Nearly in the center of the figure is seen a small, irregular, thick-walled cavity (p); this is the pituitary body, and its connection with the roof of the pharynx may easily be made out in another section. The mesoblast in this region of the sections contains numerous large and small blood-vessels and exhibits certain denser areas which probably represent the beginnings of the cranial cartilages. No sign of the forebrain is seen (the plane of the section passing in front of that region), except that the tip of the wall of one of the cerebral hemispheres (ch) is cut. The left nasal chamber (n) is shown: it will be noted again in the following section. The eye on the right side shows no remarkable features; its lens (ln) is large and lies well back of the lips of the optic cup, which may now be called the iris (ir). A thin layer of mesoblast has pushed in between the lens and the superficial ectoderm to form the cornea, and the outer wall of the optic cup is now distinctly pigmented. The inner wall of the optic cup is beginning to differentiate into the retinal elements. The eye on the left side is cut farther from its central region and has a very different appearance from the eye just described. This unusual appearance is due to the fact that the section passed through the choroid fissure, which is very large and seems to be formed by the pushing in of the walls of the cup and not by a mere cleft in these walls. This fissure is hardly noticeable in the stage preceding the present, and in a stage slightly older it has disappeared; so that it would seem to be a very transient structure. It apparently is formed at about the time that the optic stalk, as such, disappears. It is in the region of the choroid fissure, if not through it, that the optic nerve (on) enters the eye. Through the fissure also enters a vascular tuft of mesoblast (pt) which may be seen projecting into the optic cup after the disappearance of the fissure. This loop of blood-vessels is doubtless the pecten.

[Figure 20d] represents a section through the hindbrain (hb), pharynx (ph), and tip of the snout. On either side of the hindbrain are a convoluted auditory vesicle (o), and several blood-vessels and nerves, while ventral to it is seen the anterior end of the notochord (nt), around which the mesoblast is somewhat more dense than elsewhere. The pharynx (ph) sends out toward the surface a narrow gill cleft (g′) in the neighborhood of each auditory vesicle. These clefts connect with the exterior by very narrow slits, not seen in the plane of this section. The opposite end of the pharynx, as seen in this figure, opens on the left (pn) into the nasal chamber. The nasal cavity on the right is cut in such a plane that it shows neither its external nor its pharyngeal opening. The nasal passages are here fairly long and nearly straight chambers; their lining epithelium is quite thick in the middle region, but becomes thinner where it merges into the epithelium of the pharynx at one end, and into the superficial epithelium at the other end. The unusual appearance of the eye (e), on the right side of the figure, is due to the fact that the plane of the section cut tangentially through the extreme edge of the eye in the region of the choroid fissure.