EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 73 
endoderm have become separated from it, a cavity arises, the 
lower surface of which becomes the entodermic shield, the 
upper surface the inner lining of the amnion, which is thus a 
closed cavity from the very beginning.! 
Cases of a less extreme nature are met with in other 
Rodents, and have been described in detail by Selenka 
(85, 784) and others. Thus for mouse and rat, for Arvicola 
and others, the cavity which in later stages is called the 
amnion 1s never in free communication with the space outside 
the trophoblast, but is always intra-trophoblastic. There is 
thus no necessity for a gradual meeting of folds in order to 
deliminate the amnion cavity which is an enclosed space 
from the very earliest (Figs. 27, 28). What is known and 
figured as the head-fold and tail-fold of the amnion in the 
mouse (Fig. 28) must be more fully investigated before com- 
paring it with the same structures in Sauropsida. Moreover, 
these folds in the mouse, when once confluent, do not form a 
boundary between the amnion cavity and the outer world, but 
between two cavities inside the trophoblast, one of which is 
the amnion. Among the Redents the case of Lepus is parti- 
cularly instructive. I have already called attention to it in an 
earlier publication (958), and have there demonstrated that 
the so-called Rauber cells form part of the trophoblast, with 
which they are continuous (Fig. 23). In Lepus the tropho- 
blast does not open out to bring the embryonic ectoderm -to 
the surface, but the trophoblastic cells above the latter flatten 
out and disappear in the way of the “‘ Deckschicht” of the 
Amphibia. ‘This is not a primitive but probably a secondary 
arrangement, as may also be inferred from what Lee has 
found in another rodent (Ammospermophilus), which may be 
1 In former publications (’95 8, p. 25) I have more than once suggested 
that the amnion formation in man and monkeys (not known by actual obser- 
vation) was probably of the same order as that of Cavia. While correcting 
this proof I became acquainted with the interesting early human ovum which 
was demonstrated at the Berlin Congress of Zoologists in April, 1908, by Drs. 
Bryce and Teacher, and I feel confident that it goes a long way towards con- 
firmation of this suggestion, if later finds will prove it to have been normal. 
