356 Papers from the Department of Marine Biology. 



(7) The lumen of the esophagus is still occluded just behind the 

 laryngeal orifice at the 32-day stage and no doubt remains stenosed 

 until near the close of the incubation period (56 days). 



(8) All forms in which an extensive atresia of the esophagus has 

 been reported (elasmobranchii, bony fishes, amphibians, certain rep- 

 tiles, chick) have the gut open to a huge yolk-sac during the greater 

 portion of the incubation period. 



The above-enumerated facts would seem to show conclusively that 

 in the Caretta embryo the temporary atresia of the esophagus is a 

 normal phase of the developmental process. This is probably true 

 also of all forms with large yolk-sacs. The occasional esophageal 

 constrictions, stenoses, and atresias in the pig embryo, other mamma- 

 lian embryos, and in man (a normal phenomenon according to Kreuter, 

 abnormal according to Lewis) probably have their explanation in 

 terms of their reptilian ancestry. These conditions in mammals are 

 apparently very variable and any discussion as to their normality or 

 abnormality in these embryos seems of small value. Congenital 

 stenoses or atresias of the esophagus in humans, as Kreuter has already 

 suggested, represent most likely a persistence or exaggeration of a 

 normal phase of embryonic development. 



A suggestion of the teleological significance of this temporary 

 atresia of the esophagus in Caretta may be obtained by seeking to 

 disclose what is actually accomplished by the phenomenon. Ob- 

 viously, it closes the respiratory anlage against the more solid contents 

 of the gut during the greater portion of the incubation period. That 

 the relation of the obliteration of the lumen of the esophagus is close 

 to the development of the respiratory anlage is strongly indicated by 

 the following facts: 



(1) Its first appearance about the time the laryngo-tracheal groove 

 begins to be separated from the esophagus and at or just behind the 

 later orifice of the larynx. 



(2) The extension of the atresia even into the glottis. 



(3) The persistence of the atresia orally until nearly the end of the 

 incubation period, when the lung is already greatly developed, and 

 the yolk is almost entirely digested. 



It is difficult to avoid the inference that the temporary solidification 

 of the esophagus is a device to protect the developing lung against the 

 invasion from the gut of yolk-globules. The respiratory anlage, being 

 a derivative of the primitive esophagus, is originally lined by an epithe- 

 lium which retains in part the ability to digest yolk material ; but soon 

 after, as it becomes more and more differentiated into the respiratory 

 type of entodermal epithelium, it must more and more lose the capacity 

 for digesting crude yolk material. It seems reasonable to suppose 

 that large quantities of yolk-globules within the pulmonary anlages 

 would seriously interfere with their normal development. That this 



