10 A. A. W. HUBRECHT. 
along another path, along which the amniotic cavity has 
from the first remained a closed vesicle. 
A very important case is that of Tupaja in which the 
embryonic shield is originally quite bent upon itself (Fig. 
30), convexity inwards, and gradually expands (under rupture 
and dehiscence of the trophoblast) into a flat surface with no 
trophoblastic covering above it (Fig. 52) by successive stages 
as they are reproduced in the accompanying diagrams. This 
arrangement possesses suggestive points of comparison with 
what has been called by Selenka (’00 a, p. 201) the “ Enty- 
pie” of the embryonic shield, such as it exists in many 
rodents. All these cases are variations upon a_ similar 
theme as that of Tupaja, and not necessarily (as Selenka 
would have it) a consequence of the blastocyst undergoing its 
development in a cavity of exiguous dimensions, to the walls 
of which it had early adhered. ‘l'upaja at once does away 
with this argument (Hubr., 799 B, p. 173), because here the 
blastocyst, while free from any attachment to the uterine 
walls, has yet the appearance of Figs. 830 and 31. The causes 
of the folded condition of the embryonic shield can hardly be 
so simply mechanical as Selenka supposed. They remain 
obscure for the present and will come anew under considera- 
tion when the origin of the amnion will be discussed. 
The point to which the facts here brought forward have 
led us is the recognition that during the development of the 
mammalian blastocyst the trophoblast, which originally 
encloses the embryonic knob, behaves very differently in 
rarious mammals at the period that out of this knob arises 
the embryonic shield with its didermic arrangement of the 
cells out of which the embryo is going to be built up. In 
the hedgehog (Figs. 37, 38), in Gymnura, in Pteropus (Figs. 
67, 68), and in the other bats hitherto examined (Fig. 8a), 
in Galeopithecus (Figs. 41, 42), in many rodents (Arvicola, 
Mus, Cavia, Figs. 24—28), in monkeys, and (most probably) 
in man the trophoblast remains an entirely closed vesicle, 
inside of which the ontogenetic development of the em- 
bryonie knob will follow its course. In other genera of 
