Atresia of the Esophagus in the Embryo of the Loggerhead Turtle. 355 



DISCUSSION. 



A complete discussion of the phenomenon of esophageal atresia here 

 described must take into special consideration the following facts: 



(1) The inception of the atresia at the 12-day stage is coincident 

 with the early stages of the formation of the respiratory anlages. 



(2) The initial point of stenosis is spatially very closely related to 

 the orifice of the developing laryngo-tracheal tube. 



(3) The transient solidification of the esophagus progresses in a 

 caudal direction, while the reestablishment of patency progresses in 

 the opposite direction. 



(4) The early stenosis of the esophagus, from the eleventh to the 

 twelfth day, results largely from an alteration in the shape of the 

 esophagus, from a tube approximately circular in cross-section to one 

 flattened dorso-ventrally into a rectangular structure with at first a 

 slit-like lumen and later a minute central circular aperture; the stenosis 

 becomes an atresia at the thirteenth day through a fusion of the dorsal 

 and ventral walls, the cells involved becoming converted into a mesen- 

 chyme-like syncytium; this process is assisted by the increase in the 

 number of cell-layers dorsally, where mitotic activity is extensive; 

 the factors involved in the change of shape of the esophagus are 

 largely mechanical, incidental to inherent and extraneous tissue growth, 

 the chief element in forcing the growth of the esophageal tube into an 

 adaptive rectangular form being most probably the denser bilateral 

 mesenchymal plates by whose medial approximation and fusion the 

 original esophageo-respiratory anlage becomes converted into the 

 esophageal and tracheal tubes. 



(5) The esophageal atresia involves also a transient stenosis and 

 atresia of the oral end (glottis) of the respiratory anlage. 



(6) The definitive perforate esophagus becomes established by a 

 process involving the transformation of certain "intercellular" spaces 

 in the central syncytium into spherical vacuoles, and the enlargement 

 and confluence of these to form larger irregular areolse; at this stage 

 (32-day) the relatively wide esophageal lumen is spanned by delicate 

 nucleated septa; these septa are ultimately drawn into the expanding 

 peripheral wall and here incorporated with practically no tissue 

 degeneration; the earlier spherical shape of the vacuoles and the dis- 

 position of the nuclei around these spaces indicate their formation 

 under the influence of an internal fluid pressure, but no coagulum 

 appears in fixed and stained tissues ; the processes of later vacuolization 

 and incorporation of the intra-luminal septa by the peripheral wall 

 are probably largely the results of the growth expansion of the eso- 

 phageal tube resulting from active proliferation of the lining-cells and 

 assisted by the now very loose and vascular enveloping mesenchyme, 

 the primitive submucosa of the esophagus. 



