As already described, the obliteration of the esophageal lumen (contrary to Kreuter) is only partly the result of excessive cell pro liferation, this being limited to the dorsal wall. The chief factor is a change in form of the cylindric tube, effected by the growth of the esophagus operating against the confining plates of denser mesenchyme, which brings about the separation of the ventral portion of the original esophageo-tracheal anlage to form the laryngo-tracheal tube. The thickened dorsal and the ventral walls of the esophagus are thus brought into apposition and finally fuse, thus obliterating the lumen by a central syncytium.
Contrary to what one might expect a priori, the lumen of the esophagus is reestablished without any tissue degeneration. This statement is in agreement with the observations both of Kreuter and of Lewis for human embryos. Lewis believes that the constricted lumen of the esophagus in the human embryo becomes enlarged by a process involving a shifting of mitotic activity from the central layer of cells to the peripheral layers, with vacuoles forming as incidental inter mediate phenomena. In the Caretta embryo there appears no striking evidence to indicate that such a process enters as a large factor. It is true that in the stages preceding the atresia mitotic figures are most numerous centrally, and again that during the reopening of the lumen mitosis is extensive among the peripheral cells; but mitosis is by no means limited to these regions, and the relatively slight excess in one region or the other at the different periods seems to me quite insufficient to account for the early closure and the later vacuolization of the esophagus in the Caretta embryo.
The chief factor in the closing process is the change in shape of the lumen, which brings the dorsal and ventral walls into contact and results in a fusion. And the main factor in producing the early vacuolization would seem to be the collection of fluid in the "inter cellular" spaces of the central syncytial mass. The earlier vacuoles are uniformly spherical and the cells are arranged about them in the manner of an epithelium, such as would result if drops of fluid grew in size among compacted cells. The irregular condition of the later vacuolization indicates, however, that another factor now becomes chiefly active--namely, one producing traction upon the walls of the vacuoles from without—thus changing them into more or less delicate and anastomosing septa. This factor is no doubt inherent in the
growth of the circumference of the esophagus, the mechanism being enabled to exert a maximum expansive effect by reason of the now very loose, wide, and vascular enveloping mesenchyme, the primitive tela submucosa. In agreement with the observations of Kreuter and of Lewis in the case of human embryos, the vacuolisation of the eso phageal epithelium involves no tissue degeneration in Caretta. Occa sional nuclei of the septa are in mitosis, and the whole meshwork, including even the most delicate trabeculat, is ultimately drawn into the peripheral epithelial wall and thus incorporated among the ento dermal cells.
1. A series of 26 loggerhead-turtle embryos, ranging from the second to the thirty-second day of incubation, were available for this study. Atresia of the esophagus is initiated during the twelfth day of incu bation. The observations are made chiefly on the 11, 12, 13, 16, 20, 25, and 32 day stages, and the conclusions are based chiefly on this selected material.
2. During the tenth and eleventh days of incubation the epithelial lining of the oral end of the esophagus (esophageo-respiratory anlage) thickens greatly dorsally, the result of extensive cell proliferation in this region. During the twelfth day the cylindric tube of the esophagus becomes compressed dorso-ventrally, thus bringing the dorsal and ventral epithelial walls in close apposition. Only the minutest central lumen persists in the oral end of the esophagus for a distance of about 0.25 mm. During the thirteenth day the oral end of the esophagus is rectangular in cross-section and completely solid for a distance of about 0.5 min. The apposed central cells have fused and have formed a plug of tissue, essentially a mesenchyme-like syncytium.
3. The initial point of atresia is over, or just behind, the orifice of the separating laryngo-tracheal anlage; and its inception is coincident with the earliest stage in the division of the original esophageo respiratory anlage into an esophageal and a laryngo-tracheal tube. By the sixteenth day the atresia has extended into the orifice of the larynx, due in part perhaps to pressure exerted by the lateral arytenoid swellings.