6 LILIAN SHELDON. 
is hardly discernible, owing to its great thinness; a few very 
flat nuclei can, however, be distinguished init. The ovum at 
this stage is completely surrounded by its thick shell, and I 
was not able to observe any micropyle or break in the latter 
for the entrance of the spermatozoon. Apart from the fact 
that the egg is oval in shape, and consequently possesses a long 
and a short axis, there was no means of distinguishing any 
difference in the surfaces, or any pole of impregnation. 
This is the oldest ovum which I have found in the ovary. 
I suppose that it now passes into the cavity of the ovary through 
the stalk, which must acquire a cavity for the purpose. The 
only indication of such a process which I have observed is that 
in some cases the stalk has a vacuolate structure, and is irre- 
gularly two cells thick. In fact, I have never found an ovum 
either in process of passing into or in the ovary, but the next 
stages of ova are in the uterus. From this I infer that the 
process must be one of extreme rapidity, as I have ovaries 
which were preserved at various times through April. 
Since, as has been already stated, spermatozoa are present in 
great abundance in the ovary and in no other part of the 
female organs, it may be assumed that fertilization of the ovum 
takes place in its passage through the ovary, and hence I have 
never observed the process, although, as I shall describe 
shortly, I have several ova in which the male pronucleus is 
clearly seen. 
The Uterine Ova.—There is a very Remanlcaile difference 
in size between the oldest ovarian ovum and the youngest uterine 
one. The greatest length registered for the former is *26 mm., 
while the smallest ovum found in the uterus measured ‘6 mm., 
or more than double. An analogous but even greater pro- 
portional difference is stated by Mr. Caldwell (6) to occur 
in Monotremata, where he alleges it to be due to the absorp- 
tion of fluid from the uterine wall, and probably it is produced 
by the same cause in the case of Peripatus. 
I have two uterine eggs in which the nucleus was absent. 
In one of them the shell adhered closely to the protoplasm of 
the egg, and was not separated from it by a space as is the 
