Atresia of the Esophagus in the Embryo of the Loggerhead Turtle. 351 



tion. The reestablishment of the lumen begins caudally and extends 

 forwards, the last point of opening being thus at the laryngeal level. 

 The opening is accomplished by a process of vacuolization of the 

 epithelial lining. Judged by the time occupied in the effective 

 vacuolization of the caudal portion of the solid esophagus, it seems 

 reasonable to suppose that the laryngeal end remains closed until, or 

 near, the time of hatching. A matter of cardinal importance concerns 

 the fact that the first point to close in the primitive esophagus is 

 medially at or just behind the level of the tracheal (laryngeal) connec- 

 tion, and the last point to open is just behind this same level. It 

 would seem, on the basis of this observation, as if the object of the 

 transient solidification of the esophagus was to close the opening of the 

 trachea during the greater period of the embryonic development of the 

 lung against the mid- and hind-gut as if to protect the lung against 

 materials contained in the gut-tube, chiefly yolk-globules. 



It remains to describe in detail the process of closure and the subse- 

 quent vacuolization of the atresia whereby the lumen becomes re- 

 established. For this purpose embryos of the eleventh, twelfth, 

 sixteenth, and thirty-second days will be chiefly employed. In the 

 light of these additional details the hypothesis above suggested, namely, 

 that the solid esophagus in the turtle embryo has its meaning in rela- 

 tion to the developing lung, perhaps protecting it against the entrance 

 of yolk and other material from the gut, will be discussed; and an 

 attempt will be made to correlate conditions in the turtle with those 

 described for other vertebrate forms, and to interpret the occasional 

 esophageal stenosis in mammalian embryos in terms of a reptilian 

 ancestry. Exactly the same series of stages, as indicated by the 

 histologic structure, occurs in the esophagus of the 32-day embryo 

 in a cephalo-caudal direction, as appears in the greater extent of the 

 esophagus of embryos of successive stages of development as repre- 

 sented in the 11- to 25-day stages. 



The esophagus of the 11-day embryo, as stated above, is open 

 throughout. But the lumen has become considerably narrowed by 

 reason of an increase in the number of layers of the lining-cells. The 

 number of layers dorsally varies from seven to nine, cell boundaries 

 being indistinct. The two outermost and the two innermost layers 

 consist of cylindrical cells; the remaining layers comprise chiefly 

 polyhedral and spheroidal cells. Division, both mitotic and appar- 

 ently amitotic, is most active among the cells of these intermediate 

 layers. The number of cell-layers ventrally and lining the laryngo- 

 tracheal groove, as indicated by the number of layers of nuclei, is four; 

 but this portion of the epithelium is apparently for the most part a 

 pseudo-stratified columnar type. Mitotic figures are most numerous 

 among the central layers of nuclei that is, next the lumen. 



