74 A. A. W. HUBRECH®. 
said (Fig. 15) to be intermediate between the entypie, such 
as it is represented in many rodents, ungulates, and insec- 
tivores (Figs. 13, 17, 29—32), and the flat embryonic ecto- 
derm of Lepus, Sorex, and others. 
A case which as far as the amnion is concerned offers great 
similarity to that of the Guinea-pig is that of the flying 
dog (Pteropus), where Selenka and Gohre (92) noticed a 
closed amnion from the very first. It develops into the 
definite amnion by a simple process of extension and moulding, 
and exists as a closed sac of ectodermal constitution long 
before a mesoblastic layer comes to duplicate its wall (Figs. 
22, 71—73). 
A very similar arrangement is met with in the yet much 
more primitive Galeopithecus (Figs. 41, 42), which, however, 
I have not yet found occasion to describe in detail. 
Another very instructive case is that of Hrinaceus and 
Gymnura. In one of these two genera we have described 
how the early blastocyst is characterised by the possession of 
an embryonic knob from which the entoderm is so quickly 
separated off that the details of its earhest development have 
not yet come to light quite sufficiently. But at the same 
time the remaining embryonic ectoderm takes somewhat 
more time to become separated off from the trophoblast than 
in other mammals. And when it does it is by the appear- 
ance of a cavity between what is going to be the ectodermal 
shield and what will remain the trophoblast that the amnion- 
cavity is heralded into existence (Figs. 33—37). Here 
again it is a cavity that appears as such that strikes us as the 
dominant feature in the phenomenon. I have elsewhere 
demonstrated (95 B) that if we have to choose what is the 
more probable earliest appearance of the amnion, as a closed 
cavity or as set of folds by the meeting of which a cavity 1s 
going to be enclosed above the dorsal surface of the embryo, 
there is undoubtedly—speaking as an evolutionist—a heavy 
1 The exact details of the formation of the amnion in Ammospermophilus 
with respect to the exact derivation of its inner (epiblastic) layer should be 
looked forward to with interest. 
