EARLY ONTOGENETIC PHENOMENA IN MAMMALS. Et 
mammals that part of the embryonic knob which is going 
to be the ectoderm of the embryonic shield rises to the 
surface, interpolates itself between the trophoblast cells, 
which then form no longer a closed sphere, but one 
that is discontinuous by the fact that at one pole this 
ectodermic shield has replaced what were originally tropho- 
blast cells. This displacement may come about as it 
does in Tupaja (Figs. 29—32) where the unfolding of the 
embryonic shield bursts open the trophoblastic covering 
above the shield, thus increasing the surface of the vesicle 
by an area which is not trophoblast, but embryonic ecto- 
derm. Or it may happen that a similar but less distinct 
process of dehiscence interpolates embryonic ectoderm in the 
trophoblastic vesicle in the way it comes about in the 
Opossum (Fig. 5), Tarsius (Fig. 20), Cervus (Fig. 13), 
Sus (Fig. 17), Ovis (Fig. 16). Or finally the trophoblast 
may continue to cover the embryonic ectoderm as in the 
case first named, but without the development of any 
cavity between it and the embryonic shield (Fig. 15). 
In this latter case, of which the classical example is the rabbit, 
as it was so clearly figured by Kolliker (Fig. 23), the tro- 
phoblast cells covering the embryonic ectoderm flatten out 
considerably, and finally disappear. Another example of 
this is the shrew (Hubrecht, 790; Fig. 26). ‘These flat 
cells—superposed to the embryonic ectoderm—were for a 
long time designated as Rauber’s cells, Rauber having been 
the first to direct attention to them. It was, however, not 
observed by Rauber, as it was later so clearly noticed by 
Kolliker, that this layer is merely the continuation of the 
peripheral trophoblast cells, but it remained for a long time 
an accepted, though erroneous interpretation, that the 
embryonic ectoderm was uninterruptedly continued in the 
peripheral trophoblast, and that Rauber’s cells were an 
additional arrangement. This error was a natural conse- 
quence of a comparison, on a false basis, hereafter to be 
corrected, of the mammalian with the avian and reptilian 
blastocyst. The opinion of certain authors (Balfour, 
