358 Papers from the Department of Marine Biology. 



chiefly active — namely, one producing traction upon the walls of the 

 vacuoles from without — thus changing them into more or less delicate 

 and anastomosing septa. This factor is no doubt inherent in the 

 growth of the circumference of the esophagus, the mechanism being 

 enabled to exert a maximum expansive effect by reason of the now 

 very loose, wide, and vascular enveloping mesenchyme, the primitive 

 tela submucosa. In agreement with the observations of Kreuter and 

 of Lewis in the case of human embryos, the vacuolization of the eso- 

 phageal epithelium involves no tissue degeneration in Caretta. Occa- 

 sional nuclei of the septa are in mitosis, and the whole meshwork, 

 including even the most delicate trabeculae, is ultimately drawn into 

 the peripheral epithelial wall and thus incorporated among the ento- 

 dermal cells. 



SUMMARY. 



1. A series of 26 loggerhead- turtle embryos, ranging from the second 

 to the thirty-second day of incubation, were available for this study. 

 Atresia of the esophagus is initiated during the twelfth day of incu- 

 bation. The observations are made chiefly on the 11, 12, 13, 16, 20, 

 25, and 32 day stages, and the conclusions are based chiefly on this 

 selected material. 



2. During the tenth and eleventh days of incubation the epithelial 

 lining of the oral end of the esophagus (esophageo-respiratory anlage) 

 thickens greatly dorsally, the result of extensive cell proliferation in 

 this region. During the twelfth day the cylindric tube of the esophagus 

 becomes compressed dorso-ventrally, thus bringing the dorsal and 

 ventral epithelial walls in close apposition. Only the minutest central 

 lumen persists in the oral end of the esophagus for a distance of about 

 0.25 mm. During the thirteenth day the oral end of the esophagus 

 is rectangular in cross-section and completely solid for a distance of 

 about 0.5 mm. The apposed central cells have fused and have formed 

 a plug of tissue, essentially a mesenchyme-like syncytium. 



3. The initial point of atresia is over, or just behind, the orifice of 

 the separating laryngo-tracheal anlage; and its inception is coincident 

 with the earliest stage in the division of the original esophageo- 

 respiratory anlage into an esophageal and a laryngo-tracheal tube. 

 By the sixteenth day the atresia has extended into the orifice of the 

 larynx, due in part perhaps to pressure exerted by the lateral arytenoid 

 swellings. 



4. The chief factor in the temporary closure of the originally open 

 esophagus is the change in shape of the esophagus from a tube approxi- 

 mately circular in cross-section to a structure of wide rectangular form 

 with at first a slit-like lumen and finally a minute central aperture. 

 The cause of the change in shape, upon which the obliteration of the 



