70 A. A. W. HUBRECHT. 
Cuaprer LIT. DrpLorropHoBLAST= SEROUS (SUBZONAL) MEMBRANE, 
Corton, Amnion, ALLANTOIS AND UmpinicaL VESICLE IN 
ONTOGENY AND IN PHYLOGENY. 
In the second chapter of this treatise we have discussed 
the facts that have convinced us, that the very early phases 
in the development of mammals are characterised by the pre- 
sence of what we look upon as a very early larval envelope : 
the trophoblast. We have on p. 15 tried to imagine how 
this larval envelope might have evolved out of arrangements 
that are met with among invertebrates. 
We have also hinted at a possible simplification of the 
current views concerning the phylogeny of the foetal en- 
velopes of the higher Vertebrates, views which, at present, 
are neither satisfactory nor unanimously accepted. 
But we have not yet given any detailed account as to how 
we have to picture to ourselves the phylogenetic processes 
by which out of this primitive trophoblast the different foetal 
envelopes have evolved, and how it has come about that some 
of those have been vascularised in diverse ways and have 
thus laid the foundation for the phenomena of placentation, 
so intimately related to the higher development which charac- 
terises the mammalia as against the lower Vertebrates. 
I must begin by stating that much of what is going to be 
brought forward in this chapter is an attempt at bringing 
together in the light of an hypothetical interpretation facts 
that have up to now been insufficiently understood or over- 
looked. A solution that is anything else than hypothetical 
can, of course, never be reached. 
1. Chorion and Amnion. 
The amnion is a membranous envelope which we encounter 
in all Mammalia and Sauropsida and of which no traces are 
found amongst Amphibia and Fishes, the two latter being 
distinguished as Anamnia from the former, the Amniota. 
Various views have been expressed concerning the phylo- 
