266 
ZOOLOGY: H. E. JORDAN 
tory 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. 
3. 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 
lumen largely depends, is the combination of growth within the esopha- 
gus in opposition to the denser lateral mesenchymal plates, by the in- 
vasion and medial fusion of which the laryngo-tracheal groove be- 
comes converted into a tube and incidentally separated from the esopha- 
gus distally. This process is assisted, as concerns the obliteration of 
esophageal lumen, by the active cell proliferation in the dorsal wall of 
the esophagus. 
4. In the sixteen day embryo, the atresia of the esophagus extends 
through about 1500 microns. Beyond the oral end vacuoles begin 
to form in the lining epithelium. These represent dilated ^intercellular' 
spaces chiefly within the central syncytial plug of tissue. They in- 
crease in number, and enlarge caudally, where they become confluent. 
During succeeding stages this process of vacuoKzation continues, until 
at the thirty-second day stage only the extreme oral end of the esopha- 
gus remains closed. 
5. Both the closure and the reestablishment of the lumen of the 
embryonic esophagus involve mechanical as well as growth processes, 
but are normal for a certain stage of the embryonic development. The 
closure is not largely dependent upon intrinsic cell division; and the 
fenestration process involves no tissue degeneration or resorption. The 
level of initial closure and the level of final perforation are approximately 
the same, namely, the laryngeal level of the esophagus. 
6. In the process of vacuolization upon which the opening of the 
temporarily stenosed esophagus depends, the larger spherical vacuoles 
are drawn into irregular areolae as if through traction exerted from 
without. This traction no doubt inheres in the growing and expand- 
ing periphery of the esophagus. The esophagus now has a fenestrated 
appearance in section; its lumen is spanned by more or less delicate 
nucleated septa which may anastomose, giving to the whole the ap- 
pearance of a wide-meshed syncytium. Ultimately the trabeculae are 
drawn into the lining epithelium, and their nuclei incorporated among 
the entodermal cells of the mucous lining. 
