ZOOLOGY: H. E. JORDAN 
263 
Oppel^. In cyclostomes the esophagus remains patent throughout 
development (Kreuter^). In certain amphibia (Bufo; Rana) the 
esophagus becomes occluded, in part through the medium of contribu- 
tory yolk globules (Meuron^) ; and the same is true for certain rep- 
tiles (Anguis fragilis, Oppel;^ Lacerta, Meuron^). According to 
Meuron^ the esophagus of the chick embryo of the fifth day is oc- 
cluded for a length of 115 microns, but regains partial patency again 
in the sixth day through the appearance of vacuoles. 
Kreuter^ was the first to describe an epithelial obliteration of the 
esophageal lumen in the human embryo; contrary to the teaching of 
Kollmann'' and other embryologists that no solid stage of the esopha- 
gus occurred in mammals and in man. Kreuter^ describes also 
similar obliterated areas in the mid- and hind-gut of embryos between 
the fourth and tenth weeks. In four human embryos, measuring from 
8.4 to 16 mm., Lewis^ describes an esophagus whose lumen is per- 
vious throughout. He, however, describes vacuoles in the epithelial 
lining of these stages similar to those described by Kreuter as stages 
in the opening of the solid esophagus. But he regards an atresia of 
the esophagus in the human embryo as abnormal at all stages (p. 368). 
It would seem that an embryonic normal atresia of the esophagus 
is a widespread phenomenon among vertebrates, and is essentially 
similar from elasmobranch fishes to man. 
The phenomenon has not yet, as far as I am aware, been described 
for turtles, a circumstance which adds to the interest of this investi- 
gation. Nor has its intimate spatial relationship to the respiratory 
anlage, and its probable functional significance, been hitherto pointed 
out. The more important results may be summarized as follows: 
1. During the tenth and eleventh days of incubation the epithelial 
lining of the oral end of the esophagus (esophageo-respiratory anlage) 
thickens greatly dorsally, the result of extensive cell proliferation in 
this region. During the twelfth day the cylindric tube of the esophagus 
becomes compressed dorso-ventrally, thus bringing the dorsal and ven- 
tral epitheHal walls in close apposition. Only the minutest central 
lumen persists in the oral end of the esophagus for a distance of about 
0.25 mm. During the thirteenth day the oral end of the esophagus 
is rectangular in cross section and completely solid for a distance of 
about half a milHmeter. The opposed central cells have fused and 
formed a plug of tissue, essentially like a mesenchymal syncytium. 
2. The initial point of atresia is over, or just behind, the orifice of 
the separating laryngo-trachea anlage; and its inception is coincident 
with the earliest stage in the division of the original esophageo-respira- 
