STUDIES IN MAMMALIAN EMBRYOLOGY. 357 
seen by Heape in the mole, by myself and by Keibel in the 
hedgehog, and in a somewhat more distant manner to what is 
figured by Selenka on pls. xv and xvi of his treatise on the 
‘ Inversion of the Germinal Layers’ for Arvicola arvalis. The 
same process leads up to the much more extreme cases of 
‘bending inwards of the embryonic epiblast, such as it is 
observed in other Rodents (cf. Selenka, |. c., pl. xvi). Heape 
was the first to draw a parallel between what he observed in 
the mole and what obtains in Rodents in this respect (‘ Quar- 
terly Journal of Microscopical Science,’ 1883, p. 448). 
Frommel’s plates give no decisive evidence on this point, and 
in his text his interpretation of the relation between the blas- 
tocyst and the uterine tissue is different. Still, if we look at 
his fig. 4 (pl. i), figs. 3 and 6 (pl. 11), there can be no doubt 
that these figures are suggestive, and that, supposing the 
connecting strings which Frommel figures between the epi- 
blast and his “ Uterusepithelium”’ to be indeed indications of 
the exfoliating process leading to the separation of U.H. and 
D.S. (our trophoblast) from the epiblast of the germinal area 
situated underneath, there would certainly be in the bat a 
close resemblance in this respect to the processes above 
alluded to that have been observed in other mammalian orders ; 
and that are, for example, figured on Pl. XVI, figs. 13—17. 
However that may be, I have here only made this digres- 
sion further to indicate how I wished to account for Masius’ 
misunderstanding of a certain point in Keibel’s and my own 
statements, and I do not wish to commit myself to drawing 
conclusions from van Beneden’s short account, which it was 
perhaps never his intention to convey. 
Returning to Masius’ account of the development of the 
placenta in the rabbit, I cite the following passage from his 
summary of results: 
‘*‘ Before the fixation of the blastocyst against the mucosa, 
two layers can be distinguished in the embryonic epiblast; a 
deeper one with cylindrical cells, a more superficial one, more 
irregularly arranged and more deeply stained by carmine, 
with the nuclei grouped together into nuclear nests and never 
