MATERIAL The material consists of a series of 26 loggerhead-turtle embryos ranging from the second to the thirty-second day of incubation. It was collected in the summer of 1914 during a visit to the Laboratory of Marine Biology of the Carnegie Institution of Washington, located on Loggerhead Key, Florida. I am greatly indebted to Dr. Alfred G. Mayer for the excellent facilities afforded for securing and preserving a large number of specimens, approximately 200.
Two fixing fluids were employed almost exclusively, namely, the Zenker-formol modification of Helly and the strong chrom-aceto °runic solution of Flemming. After the Helly fixation, different sets of sections were variously stained with the Giemsa solution, the iron hematoxylin mixtures of Heidenhain, and the Delafield's hematoxylin and eosin combination.
In embryos up to the end of the eleventh day the lumen of the eeophagus is still patent, but the epithelial lining is greatly thickened dorsally and the lumen is in consequence constricted (fig. 1).' Dorsally the lining epithelium consists of about 7 layers of spheroidal cells; ventrally it is only 3 to 4 layers thick, and the cells are cylindrical. Figure 1 is taken at the level of the oral end of the laryngo-tracheal evagination. Figure 2 is taken 150 microns caudally, at the point of bifurcation of the trachea into the bronchi. These levels are important landmarks, for they are approximately the limits of the later atresia. It should be noted also that the laryngo-tracheal anlage at this stage is patent throughout; at later stages the epithelial thickening may occasionally involve also its orifice and cause a temporary occlusion. Mitosis is very extensive among the entodermal lining-cells of this region, especially centrally.
By the end of the twelfth day the esophagus is practically solid (fig. 3) from the level of the origin of the tracheal anlage to the point of its bifurcation, about 250 microns. A minute central opening, approximately the diameter of a nucleus of the surrounding cells, may remain. By the thirteenth day this opening also has become closed.
At this stage the trachea is open throughout. This stage is of prime importance, for it shows the manner of closure of the esophageal tube.
During succeeding days the extreme proximal end of the trachea (larynx) becomes solid, as well as the esophagus between the limits of the origin and bifurcation of the trachea. At the end of the sixteenth day the esophageal atresia extends through 1,680 microns, from a medial pharyngeal stenosis 300 microns behind the point of origin of the laryngo-tracheal tube to 720 microns behind the point of its bifurcation. Figure 4 is of the cephalic extremity of the atresia, and shows also the atretic condition of the trachea proximally. Figure 5 is of a section 300 microns caudal to figure 4. Vacuoles occur among the epithelial cells centrally; such are present to the point of transition into the stomach through 1,380 microns (fig. 6).
In the 25-day embryo the esophagus is closed through 2,650 microns. The initial point of closure is medially just behind the laryngeal open ing. As compared with earlier stages (e. g., the 16-day embryo), it seems that the cephalic extremity of earliest closure—i. e., over or just behind the tracheal opening—has shifted slightly caudally so as to leave the trachea (larynx) freely pervious anteriorly to the closed esophagus. The medial (pharyngeal) closure rapidly spreads laterally, so that within 100 microns the laterally wide, dorso-ventrally much compressed esophagus is completely closed, except for the lateral extremities, which contain a small central aperture. These lateral apertures persist through about 600 microns, the extremities having meanwhile bent ventrally, giving the esophagus through about 600 microns a wide crescent-shape, about 200 microns behind which point vacuoles begin to appear within the epithelial lining. These vacuoles increase in number and in size, and coalesce towards the caudal extremity. Figure 7 is taken at the point of transition from the esophagus to the stomach.