DEVELOPMENT OF THE ALBINO RAT 43 



is impossible to orient the ova prior to sectioning, the securing 

 of desirable sections is a matter of chance. The difficulty is 

 further enhanced by reason of the fact that owing to shrinkage 

 as a result of the action of the fixing fluid, the ova in the vesicle 

 stage are apt to be more or less folded, so that even though the 

 plane of section may be that desired, the resultant sections Lose 

 in value by reason of this folding. 



It has been shown that in the albino rat, the ova pass from the 

 oviduct to the uterine horn toward the end of the fourth day. 

 During the first half of the fifth day, the migration of the ova 

 from the oviduct to the uterine horn appears to be completed, 

 so that by the second half of the fifth day the ova are spaced 

 in the uterine horn about as after fixation to the uterine 

 mucosa. As to the factor or factors which play a role in 

 the descent of the ova through the uterine horn and their 

 fairly regular spacing, my own material gives no data; these 

 changes occurring, apparently, during the first half of the 

 fifth day, covering which my material is lacking. Widakowich, 

 who has given especial study to these questions, presents the 

 following considerations: In the downward migration of the ova 

 in the uterine horn, it cannot be assumed that the ova are capable 

 of active movement nor can their motion be ascribed to the action 

 of gravity. While peristaltic action may play a part, it is diffi- 

 cult to see how peristalsis could be so regulated as to space the 

 ova fairly regularly within the uterine cavity. The presence of 

 a ciliated epithelium in the human uterine cavity during the 

 intermenstrual period suggested the presence of a ciliated epithe- 

 lium in the uterine horn of the rat. After many preparations 

 had been searched in vain for its presence, Widakowuch found 

 short cilia, not more than 2 n long in the epithelium lining the 

 uterine cavity of a rat killed four days after copulation, and 

 containing ova in the blastodermic vesicle stage. It would ap- 

 pear, therefore, that the uterine epithelium of the rat presents 

 a ciliary border for only a relatively short time, and that the 

 transportation of the ova within the uterus is effected by the cilia. 

 Mandl also found, his material however not including the rat, 

 that cilia are present in many animals on the epithelium lining 

 the uterus only at certain periods, and perhaps only relatively 



