In the 32-day embryo the originally solid esophagus is fenestrated— except for the extreme anterior extent, about 100 microns, just behind the opening of the larynx—because of numerous vacuoles, which enlarge and coalesce caudally. Figure 8 is taken from the point of transition to the stomach. This portion of the esophagus has the appearance of a meshwork with large irregular areolte and delicate nucleated trabeculte (fig. 9). It suggests somewhat notochordal tissue of an embryo of about the 16-day stage. The esophagus is dosed medially in a manner similar to earlier stages, 110 microns behind the opening of the trachea (larynx into pharynx). It is completely closed except for minute lateral canals for 100 microns. The stenosed (vacuolated) area extends through 5.33 mm.
In 5 embryos of Chelydra serpentina examined the facts are as follows: In a 4.2-mm. embryo the esophagus is open throughout. In a 6-mm. embryo, which corresponds closely in development to the 13-day Caretta embryo, the esophagus is solid from a point just behind the origin of the tracheal anlage to its bifurcation, through 670 microns. In another 6 mm. embryo the solid esophagus opens again 240 microns short of the bifurcation of the trachea, extending through only 580 microns. In a 7-mm. embryo the esophagus is solid for 560 microns, from a point 150 microns behind the original tracheal evagination to a point slightly forward of the bifurcation. In another 7-mm. embryo the esophagus is closed from a point 240 microns behind the connection with the trachea to a point 80 microns forward of the bifurcation, through 800 microns. In both the 6 mm. and 7-mm. embryos numer ous vacuoles occur in the epithelial lining. Practically identical con ditions prevail in Chrysemys marginata. In the older embryos the prim itive glottis also becomes obliterated through apposition and fusion of its lateral walls.
From the above it appears that after the eleventh day of incubation the esophagus of Caretta is solid throughout a greater or less extent until at least the thirty-second day stage. This secondary imperforate area extends from about the point of the connection between the fore gut and the trachea to the point of bifurcation of the latter into the bronchi. The closure of the originally open esophagus is accomplished in part by cell-proliferation, chiefly in the dorsal wall of the esophagus, accompanied by a dorso-ventral compression of the cylindric tube into one of rectangular outline; and it progresses in a cephalo-caudal direc tion. The reestablishment of the lumen begins caudally and extends forwards, the last point of opening being thus at the laryngeal level. The opening is accomplished by a process of vacuolization of the epithelial lining. Judged by the time occupied in the effective
vacuolization of the caudal portion of the solid esophagus, it seems reasonable to suppose that the laryngeal end remains closed until, or near, the time of hatching. A matter of cardinal importance concerns the fact that the first point to close in the primitive esophagus is medially at or just behind the level of the tracheal (laryngeal) connec tion, and the last point to open is just behind this same level. It would seem, on the basis of this observation, as if the object of the transient solidification of the esophagus was to close the opening of the trachea during the greater period of the embryonic development of the lung against the mid- and hind-gut as if to protect the lung against materials contained in the gut-tube, chiefly yolk-globules.
It remains to describe in detail the process of closure and the subse quent vacuolization of the atresia whereby the lumen becomes re established. For this purpose embryos of the eleventh, twelfth, sixteenth, and thirty-second days will be chiefly employed. In the light of these additional details the hypothesis above suggested, namely, that the solid esophagus in the turtle embryo has its meaning in rela tion to the developing lung, perhaps protecting it against the entrance of yolk and other material from the gut, will be discussed; and an attempt will be made to correlate conditions in the turtle with those described for other vertebrate forms, and to interpret the occasional esophageal stenosis in mammalian embryos in terms of a reptilian ancestry. Exactly the same series of stages, as indicated by the histologic structure, occurs in the esophagus of the 32-day embryo in a cephalo-caudal direction, as appears in the greater extent of the esophagus of embryos of successive stages of development as repre sented in the 11- to 25-day stages.
The esophagus of the 11-day embryo, as stated above, is open throughout. But the lumen has become considerably narrowed by reason of an increase in the number of layers of the lining-cells. The number of layers dorsally varies from seven to nine, cell boundaries being indistinct. The two outermost and the two innermost layers consist of cylindrical cells; the remaining layers comprise chiefly polyhedral and spheroidal cells. Division, both mitotic and appar ently amitotic, is most active among the cells of these intermediate layers. The number of cell-layers ventrally and lining the laryngo tracheal groove, as indicated by the number of layers of nuclei, is four; but this portion of the epithelium is apparently for the most part a pseudo-stratified columnar type. Mitotic figures are most numerous among the central layers of nuclei—that is, next the lumen.