352 Papers from the Department of Marine Biology. 



Where the wide pharynx passes into the oral end of the esophagus 

 the lumen becomes triangular in cross-section, the base directed 

 dorsally. The esophagus at this level is lined by 5 to 6 layers of 

 cells throughout. The peripheral and central cells are cylindrical in 

 shape, those of the intermediate layers have a polyhedral form, and 

 many of the latter are in mitosis. At a slightly lower level the dorsal 

 portion of the lining epitheUum thickens, while in the ventral portion, 

 i. e., along the tracheal evagination, the epithelium becomes somewhat 

 thinned. The shape of the lumen at this level, including here in con- 

 tinuity a portion belonging to the esophagus proper and a portion 

 belonging to the laryngo-tracheal groove, is oval in transection, 

 flattened from side to side. 



At the level of bifurcation of the trachea the lumen of the esophagus 

 has a narrow, spindle-shaped outline, compressed from side to side. 

 This lumen is lined by a pseudo-stratified columnar epithelium, con- 

 taining 4 rows of nuclei dorsally and 2 to 3 ventrally. The res- 

 piratory anlage, of rectangular outline, flattened dorso-ventrally, is 

 lined by a similar type of epithelium with about 3 rows of nuclei. 

 Mitosis is active among the cells immediately surrounding the lumen. 

 Terminal bars are conspicuous, especially in the tracheal anlage. 



The ventral laryngo-tracheal groove becomes separated from the 

 primitive esophagus, and is consequently converted into a tube, by a 

 process involving the approximation and ultimate fusion of bilateral 

 mesenchymal plates, which process extends in a caudo-oral direction. 

 In the area of the advancing residual connection between the separating 

 tracheal and esophageal tubes, i. e., at the level of constriction pre- 

 ceding the separation, the cells are evidently under considerable pres- 

 sure. The nuclei are in general much smaller, of very variable size, 

 and greatly crowded. They are spheroidal in outline and many show 

 signs of degeneration. Many of these nuclei are vesicular and prac- 

 tically achromatic, except for a relatively large and very chromatic 

 nucleolus. None of these cells are in mitosis, but many of the nuclei 

 are at some phase of amitotic division. In the surrounding mesen- 

 chyme, however, especially in the plates which invaginate the eso- 

 phageo-tracheal tube in the process of separation, mitosis is very active. 

 Along the line of separation, the nuclei in the ventro-medial epithelium 

 of the esophagus and in the dorso-medial epithelium of the trachea are 

 much smaller, more nearly spherical, and more crowded than elsewhere 

 in these tubes. A few smaller spheroidal yolk-giobules are present in 

 the lining-cells of both the esophagus and trachea at these stages, but 

 they are relatively more numerous in the esophageal epithelium. 



In the 12-day embryo the esophagus practically lacks a lumen 

 (fig. 3). At the level just behind the point of connection between 

 the primitive esophagus and the trachea (definitive pharynx and 

 larynx) the narrow circular lumen of the trachea is lined with 3 to 4 



