Atresia of the Esophagus in the Embryo of the Loggerhead Turtle. 359 



lumen largely depends, is the combination of growth within the esoph- 

 agus in opposition to the denser lateral mesenchymal plates, by the 

 invasion and medial fusion of which the laryngo-tracheal groove 

 becomes converted into a tube and incidentally separated from the 

 esophagus distally. This process is assisted, as concerns the oblitera- 

 tion of the esophageal lumen, by the active cell proliferation in the 

 dorsal wall of the esophagus. 



5. In the 16-day embryo the atresia of the esophagus extends 

 through about 1,500 microns. Beyond the oral end vacuoles begin to 

 form in the lining epithelium. These represent dilated " intercellular " 

 spaces chiefly within the central syncytial plug of tissue. They in- 

 crease hi number, and enlarge caudally, where they become confluent. 

 During succeeding stages this process of vacuolization continues, 

 until at the 32-day stage only the extreme oral end of the esophagus 

 remains closed. 



6. Both the closure and the reestablishment of the lumen of the 

 embryonic esophagus involve mechanical as well as growth processes, 

 but are normal for a certain stage of the embryonic development. 

 The closure is not largely dependent upon intrinsic cell-division, and 

 the fenestration process involves no tissue degeneration or resorption. 

 The level of initial closure and the level of final perforation are approxi- 

 mately the same namely, the laryngeal level of the esophagus. 



7. In the process of vacuolization upon which the opening of the 

 temporarily stenosed esophagus depends, the larger spherical vacuoles 

 are drawn into irregular spaces as if through traction exerted from 

 without. This traction no doubt inheres in the growing and expanding 

 periphery of the esophagus. The esophagus now has a fenestrated 

 appearance in section; its lumen is spanned by more or less delicate 

 nucleated septa which may anastomose, giving to the whole the appear- 

 ance of a wide-meshed syncytium. Ultimately the trabeculse are 

 drawn into the lining epithelium and then* nuclei incorporated among 

 the entodermal cells of the mucous lining. 



8. The temporary atresia of the esophagus in the Caretta embryo 

 would appear to be a device for the protection of the lung during its 

 development against yolk material from the gut, which material could 

 not be digested, but would interfere with normal development of the 

 lung. 



9. This hypothesis can comprehend and correlate conditions in 

 embryos of forms with meroblastic, holoblastic telolecithal, and aleci- 

 thal eggs. Where yolk is very abundant, as in the meroblastic eggs 

 of fishes, reptiles, and birds, the atresia is relatively extensive and of 

 longer duration; in amphibia the closure is largely of the nature of a 

 stenosis in which yolk-globules are involved, probably in process of 

 digestion, while their forward progress is delayed by reason of the con- 



