388 ARTHUR ROBINSON, 
The hypoblast of the rat’s ovum at this stage is more dis- 
tinctly divided into cells than that of the mouse ovum (com- 
pare figs. 11 and 12), and this is more especially the case round 
the proximal part of the epiblast. 
The epiblast is 76 long and 53, broad. It contains an 
oval cavity which is situated in the centre of the mass. 
The trophoblast, which is 102 ~ long and 26 u broad, is not 
constricted where it enters the invagination cavity, but it con- 
tains in this situation a small cavity. The trophoblastic 
nuclei are arranged with some degree of order round the cavity, 
but distally and proximally they are scattered irregularly in 
the protoplasm. 
The account given by Duval (9) of the events of the seventh 
day varies somewhat from Selenka’s (44, 45) description of 
the changes which take place in ova of apparently the same 
period, and as the results of my own observations of this period 
differ materially upon many points from those of the observers 
named, it is necessary that I should give a short account of 
their results and a comparison with my own before entering 
upon a summary of the latter. 
1. The Uterine Epithelium. 
It is during the seventh day that the uterine epithelium in 
the neighbourhood of the ovum disappears. This disappear- 
ance is due to asimple atrophy, according to Selenka’s figures ; 
whilst according to Duval’s account, which is confirmed by my 
specimens, the disappearance is preceded by a degeneration. 
The outlines of the uterine epithelium cells are lost, and the 
nuclei become smaller and more numerous; subsequently both 
the protoplasm and the nuclei are absorbed. Possibly the 
cells become pabulum for the growing ovum, as Duval suggests. 
There is no proof that this is the case beyond the fact that 
the disappearance of the uterine epithelium and the increase 
of the ovum are simultaneous events, and it is quite as pro- 
bable that the uterine epithelium is absorbed by the wandering 
cells from the uterine capillaries, which have in the meantime 
become much dilated, for such cells are known to possess the 
