Atresia of the Esophagus in the Embryo of the Loggerhead Turtle. 349 



DESCRIPTIVE. 



In embryos up to the end of the eleventh day the lumen of the 

 esophagus 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 extremitj'' of the atresia, and shows also the 

 a,tretic 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 earHer 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 



'The photomicrographs were made by Mr. William S. Dunn, Cornell University Medical 

 School. New York. 



