DEVELOPMENT OF THE ALBINO RAT 45 



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 



