16 A. A. W. HUBRECHT. 
eeneralisation into the developmental history of vertebrates 
may be far-reaching, 
Up to now foetal envelopes or membranes were only known 
in the ontogeny of reptiles, birds and mammals at somewhat 
later stages of their development. These membranes were 
respectively known aS amnion, chorion, serous membrane, 
subzonal membrane (and in case of the Sauropsids and certain 
mammals, even allantois) so that Milne Edwards’ subdivision 
of the vertebrates into Amniota, Allantoidea, as against the 
Anamnia, Anallantoidea, was based on the presence or absence 
of such membranes. Of the phylogenetic evolution of these 
foetal membranes no reasonable explanation has yet been 
offered, as is, for example, recognised, as far as the amnion is 
concerned, in an unbiassed handbook of human embryology, 
as is that of Sedgwick Minot (p. 344, lst edition). Now this 
obscure phylogeny would seem to become yet more compli- 
cated when we add to the already existing foetal membranes 
a new larval envelope, called trophoblast. The case is, how- 
ever, quite the contrary. This early envelope, that we have 
seen making its appearance soon after the very first phases of 
segmentation of the mammalian ovum, instead of adding new 
difficulties, helps to explain old ones. It throws new and 
unerring light on the first origin both of the amnion and the 
chorion (respectively : serous membrane) and may prove to 
be a valuable key that may lead to a reasonablejnterpretation 
of much that is as yet obscure and incomprehensible. Out of 
this very earliest larval envelope the others seem to have 
gradually evolved; they may be looked upon as further differ- 
entiations of it and we have now to look out for the first 
origin of the trophoblast itself and see if we can furnish a 
hypothesis worthy of further consideration, In that case the 
phylogeny of the other foetal membranes would & fortiori 
have been explained at the same time. 
Now, I believe that we have only to assume that the ances- 
tors of those Vertebrates in which a distinct trophoblast or 
the traces of it are found, were already possessed of a larval 
envelope in the antecedent stages of phylogeny, in order to 
