72 stig A. A. W. HUBRECAHT. 
Erinaceus, Gymnura, monkeys and man) than one which is 
obliged to derive the amniogenesis in the latter mammals 
from the phenomena which we notice in Sauropsida and a 
number of mammals by an obscure process of cenogenesis. 
Such an alternate hypothesis I have advocated (’95B) more 
than thirteen years ago, and will here state it anew with 
certain additional facts in its support. 
The starting-point for this hypothesis, which will enable us 
to admit a separate phylogeny for chorion and for amnion, 
was the fact that we have observed in all mammals the pre- 
sence of an actual foetal envelope—the trophoblast—long 
before the appearance of anything like the am- 
nion. And secondly the other fact that this early foetal 
envelope is bodily transformed into what is called the chorion, 
diplotrophoblast, serous membrane or subzonal membrane in 
later stages of development. There can thus be little doubt 
that we now have to turn the tables and must no longer look 
upon this outer envelope as a by-product of the amnion, but 
must, on the contrary, ask how has the amnion come to be 
developed out of or by the side of the older foetal membrane 
or embryonic envelope, the trophoblast ? 
Leaving aside the interesting, but at present quite unsolv- 
able question whether vertebrates have ever existed in older 
geological times that possessed a trophoblast but were not yet 
provided with an amnion (a question which is justified as 
soon as we have established that the lmk between amnion and 
serosa js not as indissoluble as the modern embryological 
text-books will have it), we will now proceed to investigate 
such cases in which the independent development of chorion 
on one side, amnion on the other is yet particularly evident. 
Of such cases I would cite some of those that have been 
already named above and that occur in different orders of 
mammals. We begin with an extreme case as is that of 
Cavia. The trophoblast and that proliferating portion of it 
which is known as the Traiger are quite independent of the 
embryonic knob at a very early stage (see Figs. 24 and 
25). In the embryonic knob, after the mother-cells of the 
