DEVELOPMENT OF THE ALBINO RAT 289 
The lumen is not smooth, but presents numerous radially ar- 
ranged folds, certain of which are relatively deep. Essentially 
the same characteristics pertain to the mucosa of the uterus, 
soon after the beginning of gravidity. As the ova pass from 
the oviduct to the lumen of the uterus they become lodged in 
certain of the mucosal folds, and generally in certain of the 
deeper ones to be found along the anti-mesometrial border. 
I find Burckhard’s account of the form of the lumen of the 
uterine horn, of the structure of the mucosa in early stages of 
gravidity, and the lodgment of the ova, pertaining to the mouse, 
applies equally well to the albino rat. No reason can as yet be 
given as to why the ova are lodged in the mucosal folds in which 
they are found, and not in others. So far as may be ascertained 
from the sections, the particular mucosal folds in which the ova 
are found, do not differ in form and structure from neighbor- 
ing folds. It is possible that by reconstruction of the epithelial 
lining of the entire uterine horn in pertinent stages, certain 
characteristics of form and position might be revealed as pos- 
sessed by certain mucosal folds which make them especially 
favorable for the lodgment of the descending ova. Such re- 
constructions, however, have not been made. Burckhard states 
that in the mouse, about the middle of the fifth day, after the 
ova have been in the uterine cavity for a number of hours, there 
may be observed the first changes in the uterine wall. The 
changes consist primarily in a flattening of the uterine epithelium. 
In the immediate region where implantation is to occur, the 
lining epithelial cells present instead of a cylindric form, a cubic 
form. The area is sharply demarked from the surrounding 
epithelium, the transition of cubic to cylindric epithelium being 
marked by a sharp-lipped epithelial ledge. In my own material 
of the rat covering these stages, the uterine mucosa likewise 
presents shallow pits, in the immediate regions where the ova 
are lodged, lined by slightly flattened, cubic epithelium, very 
much as described by Burckhard for the mouse. Widakowich 
presents an excellent figure (fig. 2 of his contribution, rat 
four days after fertilization—‘Befruchtung’) showing clearly 
the relations of the ova to the uterine wall. In this rat, the 
