EARLY ONTOGENETIC PHENOMENA IN MAMMALS. Zi 
genesis of the amnion. And it must be recognised that the 
appearance—all of a sudden—of this useful and complicated 
foetal investment—though it is only present during the 
few weeks or months between early ontogeny and birth—is 
strictly comparable, both as regards its constituent layers and 
the way it comes into existence wherever we find it. This 
natural phenomenon must have its natural evolutional 
explanation for whosoever wishes to be guided by evolu- 
tionary principles in his interpretation of nature. The 
explanations given, and which we owe to Haeckel, van 
Beneden and others are, however, as we shall see, untenable. 
I have held this view long ago (’95), and have made another 
attempt at solving this evolutionary riddle, but find that a 
repetition of my argumentation towards an alternative solu- 
tion of this intricate problem may not seem out of place. 
Now it is a very queer point that two of the foetal enve- 
lopes, the amnion and the serous membrane, seem to be so 
intricately interlocked with each other as far as their first 
appearance goes, that the serous membrane, as Schauinsland— 
the author of the chapter on foetal membranes in Sauropsida 
in Hertwig’s new handbook—puts it, owes its origin to the 
outer pleat of the amnion fold, but later increases in size by 
a process which splits it off from the umbilical vesicle. Of 
late the name amniogenetic chorion (Bonnet) has been intro- 
duced for mammals in the place of serosa, thus underlining 
the supposition that it owes its origin to the amnion. 
We would expect a feetal envelope of the importance of the 
chorion in Primates and of the subzonal membrane or serosa 
(diplotrophoblast) in other Mammalia and Sauropsida to 
have a phylogeny of its own and not to be a by-product 
of a folding process that is typical for the Sauropsida, but is 
absent in representatives of many orders of Mammalia. 
A hypothesis which would separate the phylogeny of 
chorion (serosa) and of amnion would thus in itself appear to 
be more acceptable and might prove to be a better guide to 
the understanding of those mammals that have no amnion- 
folds (Cavia and other rodents, Pteropus, Galeopithecus, 
