The alligator and its allies

by A. M. Reese

Figures 16-16g ([Plates XXII.], [XXIII.])

The embryo ([Fig. 16]) now lies on one side, body torsion being complete. The curvature of the body is so marked that the exact length is difficult to determine. The eye (e) and ear (o) have about the same superficial appearance as in the preceding stage. The nose is not shown in this figure. About thirty somites are present; the exact number cannot be determined in surface view. The amnion is complete, though not shown in the figure, and the tail (t) is well formed. The umbilical stalk was torn in the removal of the embryo, so that it is not shown in the figure. The dim outline of the now convoluted heart may be seen if the “cleared” embryo be viewed by transmitted light; it is not shown in the figure. The allantois (al) is a rounded sac of considerable size just anterior to the tail. Four gill clefts (g¹⁻⁴) are now present; the most posterior one is more faint than is represented in the figure, and it could not be definitely determined from a surface view whether or not it opened to the exterior. The mandibular fold (md) is now fairly well outlined, but there is as yet no sign of the maxillary process.

[Figure 16a] is the most anterior of a series of transverse sections made of an embryo of the approximate age of the surface view just described; it passes through the tip of the forebrain (fb) and shows the nasal pit (n) of the right side. The great thickening of ectoderm in the region of the nasal invagination is represented by a solid line. Owing to the obliquity of the section, the left nasal pit was not cut. The mesoblast is quite dense and contains two or three small blood-vessels near the roof of the brain. The plane of this section, owing to the cranial and body flexure, cut the embryo also in the region of the pharynx; this part of the section was, as a matter of convenience, omitted from the drawing.

[Figure 16b] is in reality more anterior in position, considering the entire embryo, than the preceding; but the region of the embryo represented is more posterior, so that it is described at this point. The greatly elongated outline of the brain is due to its being cut through the region of flexure, so that the forebrain (fb), or, perhaps, midbrain, is shown at one end, and the hindbrain (hb) at the other. The walls of these cavities are somewhat wrinkled and irregular and their constituent cells are beginning to show slight differentiation, though this is not shown in the figure. On the left side are seen a couple of darkly stained masses; one is the origin of a cranial nerve (cn); and the other is one of the auditory vesicles (o), which is still more irregular in outline than it was in the preceding stage. The only blood-vessels to be seen are a few very small ones that lie close to the wall of the brain. The ectoderm is quite thin at all points.

[Figure 16c], the largest section of this series, passes through the forebrain in the region of the eyes and through the gill clefts. The forebrain (fb) exhibits on the left a marked thickening of its wall (ch), the edge of the cerebral hemisphere of that side, which is just beginning to develop; on its right side the lower part of the forebrain is connected by a well marked optic stalk (os) with the optic cup (oc), in whose opening lies the lens vesicle (lv), now reduced to a crescentic slit by the thickening of its posterior wall. The mesoblast is more dense in those parts of the section adjacent to the pharynx than in the more distant regions, and the ectoderm thickens in a marked way as it approaches the borders of the pharynx and gill clefts. Only a few small blood-vessels (bv) are to be seen in the region of the forebrain.

Parts of three pairs of clefts (g) are shown in the figure: one pair opens widely on either side, so that there is a large area of the section that is distinct from the two still larger portions and contains a small, thick-walled cavity (g) on the right side; this cavity is a gill cleft that is cut through neither its outer nor its pharyngeal opening.

No structures other than this small portion of a gill cleft and a few blood-vessels are to be seen in this middle region of the section. In the more posterior part of the section, in which the notochord (nt) is located, a pair of curved clefts may be seen, opening entirely through the wall on the left, but closed on the right (g). One distinct pair of aortic arches is shown (ar), and also the dorsal aortæ (ao), which are of very unequal size. The spinal cord (sc) and muscle plates need no special description.

[Figure 16d] is in the region of the heart (ht) and lungs (lu). The former is an irregular cavity whose walls, especially on the ventral side (mes′), are becoming very thick and much folded. Although thin, the body wall completely surrounds the heart, as would be expected, since this was true of the preceding stage. The lung rudiments (lu) and the foregut from which they have arisen have the same appearance as in the chick; they consist of three small, thick-walled tubes so arranged as to form a nearly equilateral triangle. They are surrounded by a swollen, rounded mass of mesoblast which almost completely fills the surrounding portion of the body cavity (bc). The pleural sides of these crescentic portions of the body (or pleural) cavity—that is, the boundary of the mass of mesoblast just mentioned—are lined with a thickened layer of cells, shown by the solid black lines in the figure. The lung rudiments may be traced through about fifty sections of this series, or about one twelfth of the entire series. At the dorsal angle of the part of the body cavity (bc) just described, near the dorsal aorta (ao), are two dark, granular masses (ge), which, under a higher magnification than is here used, are seen to consist of a small group of blood-vessels filled with corpuscles; although several sections in front of the anterior limits of the kidneys these are evidently glomeruli. They may be traced, though diminishing in size, far toward the tail, in close connection with the Wolffian bodies. At intervals they are connected by narrow channels with the dorsal aorta; no such connection was present in the section drawn. The notochord (nt), spinal cord (sc), muscle plates (mp), and spinal ganglia (sg) need no special mention. The mesoblast is beginning to condense in the neighborhood of the notochord, and the ectoderm is slightly thickened laterally and dorsally.

[Figure 16e] is in the region of the liver and the Wolffian bodies; it also shows the tip of the ventricular end of the heart. The liver (li) is a large irregular mass, of a blotchy appearance under this magnification, lying between the heart (vn) and the intestine (i). Under greater magnification it is seen to be made up of indefinite strings of cells; and its still wide opening into the intestine may be seen in more posterior sections. The intestine (i), which in this section might be called the stomach, is a fairly large cavity with the usual thick entodermic walls; it is supported by a comparatively narrow mesentery. The body cavity on the side next this mesentery has the same thick lining that was noted in the region of the lungs. The convolutions of the thick peritoneal lining may easily be mistaken in places for parts of the enteron. The Wolffian bodies may be seen as two groups of tubules (wt) in their usual location. The heart is cut through the ventricle (vn), as has been said. The section being at right angles to the long axes of the villi-like growths of the myocardium, the depressions between these mesoblastic cords are seen as a number of small irregular areas, each one lined with its endocardium. The incompleteness of the body wall below the heart is apparently due to an artificial break and not to a lack of fusion. The only point that need be mentioned in connection with the structures of the dorsal part of the section is that the distinctness of the myocœl (myc) on the right side is somewhat exaggerated.

[Figure 16f] is in the middle region of the embryo where both splanchnopleure and somatopleure are unfused. Owing chiefly to the unclosed condition of the midgut (i) and to the increase in length of the mesentery (ms), the section is quite deep dorso-ventrally. The continuation of the amnion (a) with the somatopleure is of course here evident.