280 LILIAN SHELDON. 



is hardly discernible, owing to its great thinness ; a few very 

 flat nuclei can, however, be distinguished in it. 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 remarkable 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 



