DEVELOPMENT OF THE ALBINO RAT 287 
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 réle 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, Widakowich found 
short cilia, not more than 2 » 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 
