52 Miss J. E. Lane- Clay pon. On the Origin, etc., of [June 16, 



ova must pass through the requisite nuclear changes. Also it is obvious, 

 although this is not a point which he brings out, that there is a very great 

 difference in size between the primordial ovum and the interstitial cell in a 

 non-pregnant animal, and it is therefore necessary for the cell to enlarge at 

 some period of the transformation. This requirement is fulfilled, as has 

 already been shown, in the case of the pregnant ovary. The interstitial cell 

 of the non-pregnant ovary has approximately a diameter of 17 n, but 

 increases up to 29 /a or even rather over 30 in the pregnant animal. The 

 size of a primordial ovum before it begins to grow, preparatory to becoming a 

 Graafian follicle, is very constant ; I have taken measurements of a large 

 number of ova both in the young ovary and in the pregnant as well as the 

 non-pregnant animal, and the average diameter is 27 /a, the diameter reached 

 by the interstitial cells about the eighteenth day of pregnancy. 



It is about the twentieth day of pregnancy that the cutting off of the 

 interstitial cells towards the periphery begins to be noticeable — that is to 

 say, shortly after they have reached a diameter about equal to that of a 

 primordial ovum. It is not, however, until a little later that the cells thus 

 cut off begin to show any nuclear differentiation, in fact this is perhaps best 

 seen in the ovary of a rabbit whose time of parturition has almost arrived. 

 These changes are identical with those taking place in the deutobroque cells 

 of the young ovary during the period of ovogenesis. The only difference lies 

 in the fact that whereas in the pregnant ovary the process is taking place 

 only at the periphery, and in relatively very small numbers, in the young- 

 ovary there may be 20 or 30 nuclei undergoing changes in the same field. 

 The fact of their presence at all in the pregnant ovary is, however, all proof 

 that is necessary for the formation of ova. It is not for a moment to be 

 supposed that any formation of fresh primordial ova after the first great 

 period should take place to anything like the same extent. Probably the 

 actual changes only occur over a period of a few days, commencing about the 

 twenty-fifth day of pregnancy, or rather earlier, and extending probably to a 

 little after parturition. In the young ovary the changes do not commence 

 until after birth, and some of the cells have completed their changes by about 

 the tenth or eleventh day, the process being probably considerably less 

 lengthy than this for the individual cells, and taking still less time, if any- 

 thing, in the pregnant rabbit, where there is obviously a state of stimulation 

 during the whole period of pregnancy. 



The first change passed through by the nucleus of an interstitial cell, which 

 has passed to the periphery in order to become an ovum, is shown in fig. 6 (1). 

 The nucleus shows chromatin filaments, in the middle of which are seen 

 irregular lumps of chromatin. (In the diagrams the analogous stage of the 



