350 Papers from the Department of Marine Biology. 



apertures persist through about 600 microns, the extremities having 

 meanwhile bent ventrally, giving the esophagus through about 600 

 microns a wide crescent-shape, about 200 microns behind which point 

 vacuoles begin to appear within the epithelial lining. These vacuoles 

 increase in number and in size, and coalesce towards the caudal 

 extremity. Figure 7 is taken at the point of transition from the 

 esophagus to the stomach. 



In the 32-day embryo the originally solid esophagus is fenestrated 

 except for the extreme anterior extent, about 100 microns, just 

 behind the opening of the larynx because of numerous vacuoles, 

 which enlarge and coalesce caudally. Figure 8 is taken from the 

 point of transition to the stomach. This portion of the esophagus has 

 the appearance of a meshwork with large irregular areolse and delicate 

 nucleated trabeculse (fig. 9) . It suggests somewhat notochordal tissue 

 of an embryo of about the 16-day stage. The esophagus is closed 

 medially in a manner similar to earlier stages, 110 microns behind the 

 opening of the trachea (larynx into pharynx) . It is completely closed 

 except for minute lateral canals for 100 microns. The stenosed 

 (vacuolated) area extends through 5.33 mm. 



In 5 embryos of Chelydra serpentina examined the facts are as follows : 

 In a 4.2-mm. embryo the esophagus is open throughout. In a 6-mm. 

 embryo, which corresponds closely in development to the 13-day 

 Caretta embryo, the esophagus is solid from a point just behind the 

 origin of the tracheal anlage to its bifurcation, through 670 microns. 

 In another 6 mm. embryo the solid esophagus opens again 240 microns 

 short of the bifurcation of the trachea, extending through only 580 

 microns. In a 7-mm. embryo the esophagus is solid for 560 microns, 

 from a point 150 microns behind the original tracheal evagination to a 

 point slightly forward of the bifurcation. In another 7-mm. embryo the 

 esophagus is closed from a point 240 microns behind the connection 

 with the trachea to a point 80 microns forward of the bifurcation, 

 through 800 microns. In both the 6 mm. and 7-mm. embryos numer- 

 ous vacuoles occur in the epithelial lining. Practically identical con- 

 ditions prevail in Chrysemys marginata. In the older embryos the prim- 

 itive glottis also becomes obliterated through apposition and fusion 

 of its lateral walls. 



From the above it appears that after the eleventh day of incubation 

 the esophagus of Caretta is solid throughout a greater or less extent 

 until at least the thirty-second day stage. This secondary imperf orate 

 area extends from about the point of the connection between the fore- 

 gut and the trachea to the point of bifurcation of the latter into the 

 bronchi. The closure of the originally open esophagus is accomplished 

 in part by cell-proliferation, chiefly in the dorsal wall of the esophagus, 

 accompanied by a dorso-ventral compression of the cylindric tube into 

 one of rectangular outline ; and it progresses in a cephalo-caudal direc- 



