354 Papers from the Department of Marine Biology. 



quent obliteration of the lumen are probably due in part also to the 

 arytenoid swellings just appearing in this region. 



Vacuoles first appear, either singly or in groups, centrally and later- 

 ally. These are the areas where the fusion-reticulum is originally 

 less dense, because here the original lumen persisted longest. As the 

 vacuoles form, the nuclei of the syncytium become disposed in the 

 manner of an epithelium around these spaces, as if rearranged under 

 pressure from within the vacuoles. The original smaller vacuoles 

 grow in size and coalesce with adjacent vacuoles. Only very rarely 

 is a mitotic figure seen among the central cells. Mitosis is sUght at 

 this stage, also among the peripheral cells. An extremely small, 

 practically neghgible, amount of cell-degeneration occurs among the 

 central cells. The vacuoles tend to become confluent in a lateral 

 direction; they are apparently empty and contain no coagulum in 

 stained sections. Their spheroidal shape and the manner of the 

 arrangement of the enveloping cells leave no room for doubt, however, 

 that they were formed, at least in large part, under the influence of a 

 fluid pressure. 



In the 25-day embryo the vacuoles are larger and more numerous, 

 and they increase in number and size caudally, where a fenestrated 

 condition of the atretic lumen has become established. 



Coincident with this fenestration, the enveloping mesenchyme 

 (tela submucosa) of the esophagus has become looser and more vascu- 

 lar, thus permitting more readily an expansion of the tube under the 

 pressure produced internally during the formation of the vacuoles. 

 This expansion is assisted also no doubt by the growth, as indicated 

 by extensive mitotic activity, of the peripheral portion of the wall, 

 and possibly in part also by external traction exerted by the growing 

 mesenchyme. 



At the 32-day stage the lumen of the esophagus is in the fenestrated 

 condition throughout its greater extent, only the most cephaUc por- 

 tion remaining practically solid. The lumen appears to be spanned 

 across by delicate nucleated septa with lateral anastomoses. Where 

 the septa join the main wall of the lumen they spread into triangular 

 multinucleated bases. There is no sign of degeneration among the 

 nuclei of these septa — in fact, a number of the nuclei may be in process 

 of either mitotic or amitotic division. The trabeculse are subse- 

 quently simply drawn into the peripheral portions of the wall and grad- 

 ually incorporated among the entodermal cells of the lining epithehum. 



