Atresia of the Esophagus in the Embryo of the Loggerhead Turtle. 353 



layers of cells, some of which, especially centrally, are in mitosis. 

 The esophagus, just cephalad of this point {i. e., in the region of the 

 glottis) has a T-shaped lumen, lined dorsally with about 9 layers of 

 cells, of cylindric form peripherally and centrally and of polyhedral 

 form in the intermediate layers. Viewed as a whole — ^. e., the lumen and 

 its cells — the esophagus is still more distinctly T-shaped, the cross-bar 

 in the esophagus proper being about equal to or even greater in length 

 than the stem or the laryngeal anlage. 



At the level here illustrated, figure 3, the minute central opening 

 of the esophagus is surrounded by only 3 to 4 layers of cells, as indi- 

 cated by the rows of nuclei. By the thirteenth day even this capillary 

 lumen is obliterated and the esophagus is completely closed for about 

 0.5 mm. This closure is apparently effected by a process of lateral 

 traction (or possibly dorso-ventral compression) upon the originally 

 cylindric tube, causing thus a change in shape of the lumen from an 

 oval or a circular to a horizontal cleft form. This mechanical factor 

 is assisted by the larger number of cell-layers dorsally, the result of 

 greater proliferative activity, and accomplishes finally a close apposi- 

 tion and ultimate fusion of the dorsal and ventral walls. One small 

 lateral vacuole was seen in the epithelial lining of the stenosed esoph- 

 agus at the 12-day stage; at the 13-day stage three lateral vacuoles 

 had appeared. 



The central area of the closed esophagus of this stage consists of 

 irregular stellate cells, resembling a compact mesenchymal reticulum. 

 These cells come chiefly from the dorsal thicker wall, and represent 

 modification products of originally cylindric cells. 



At the 16-day stage of development the esophagus is closed for 

 approximately 1.5 mm. At the most anterior point of occlusion the 

 pharynx is wide and somewhat crescentic, with the concavity directed 

 ventrally. The occluded area at this level occupies only the smaller 

 medial portion of the wide pharynx at the point where it passes into 

 the esophagus. The larynx at this level has an oval, slit-like lumen, 

 compressed laterally and lined with a pseudo-stratified columnar 

 epithelium containing 2 to 4 rows of oval nuclei, many of which cen- 

 trally are in mitosis. The pharynx is lined with columnar epithelium 

 (apparently pseudo-stratified) with 3 rows of nuclei, oval in shape 

 peripherally and centrally. At the most cephalic point of the closure 

 the pharynx remains widely open laterally. Only a few mitotic 

 figures appear among these cells. In the median fused area the 

 central cells are of irregular shape, with spheroidal nuclei, and form a 

 mesenchyme-like syncytium. At this stage the glottis is closed for 

 a short distance, due probably to the pressure exerted upon this por- 

 tion of the tube by the surrounding mesenchyme in the process of final 

 cephalic division of this portion of the fore-gut into esophagus and 

 trachea. The fusion of the lateral walls of the larnyx and the conse- 



