DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 423 
coming that those cells are of epiblastic nature. It is true 
that similar cells overlap the inner mass, but the fate of the 
overlapping cells is unknown, and it is quite possible that, as 
in the rabbit, they disappear and leave the remainder of the 
vesicle wall in continuity with the peripheral hypoblastic cells 
of the inner mass. Concerning the attainment of the dider- 
mic condition in the bat, van Beneden and Julin make a most 
siguificant statement. They say that “ V’hypoblast, que s’étale 
progressivement en partant de la tache embryonuaire & la face 
interne de l’épiblast, ne gague jamais le pole inférieur de la 
vésicale blastodermique ; une zone monodermique persiste 
jusqu’A une phase trés avancée du développement” (4, p. 
569). Now, if there is any close relationship between the 
mammalian ovum and the ova of other Vertebrates, it is much 
more probable that the epiblast extends over the surface of 
the hypoblast, leaving for a time a portion of the latter layer 
uncovered, than that the cavity of the yolk-sac is for a long 
time bounded partially by epiblast ; and if this should prove to 
be the case, then the ovum of the bat is intermediate in 
position, as regards the extension of the outer layer over the 
inner, between the ova of the rat and the mouse on the one 
hand, and the ovum of the hedgehog on the other. 
In the mole, Heape describes appearances in the early 
stages very similar to those already noted as occurring in the 
rabbit. ‘Towards the end of the segmentation period the 
ovum consists of a solid mass of cells which, in optical section, 
appear to be separable into an outer layer and an inner mass ; 
at a later stage a cavity appears, as in the rabbit, and the 
inner mass still remains adherent to a portion of the outer 
layer. Heape assumes that the outer layer is epiblastic, and 
certainly his figures (17, pl. xxix, figs. 17, 18, and 19) give the 
idea that the inner mass and the outer layer are quite dis- 
tinct; but, as Heape has shown, the outer layer disappears, as 
a layer, from over the outer surface of the inner mass by 
fusing with the latter to form the embryonic epiblast. In the 
stages which precede this fusion (pl. xxix, figs, 21, 22, and 
23) the outer layer is intimately connected with the margin 
