KARLY ONTOGENETIC PHENOMENA IN MAMMALS. 81 
myself to a comparison between it and the mammalian 
trophoblast. I have since, in a recent publication (’07, 
p- 60), more distinctly accentuated that I would never look 
' upon the Deckschicht of the Amphibia as having been the 
first starting-point of what afterwards becomes amnion and 
chorion of the higher mammals. We may safely say that 
Deckschicht and trophoblast are homologous and of similar 
descent, but we cannot at present fully picture to ourselves 
what has been the arrangement of the larval envelope in the 
common parent form from which both have derived. ‘There 
is no doubt that the viviparity in those vertebrates that have 
become the higher mammals has contributed towards making 
the trophoblast ever so much more conspicuous. But whether 
for the Amphibia we may also assume that in past times the 
trophoblast was more conspicuous in very early stages and 
enclosed an embryonic knob, such as we notice in mammals, 
cannot be decided for the present. Suffice it to say that some 
few observations would seem to point in this direction. I do 
not, however, wish to develop these at present, considering 
the very hypothetical nature of the ground we are here 
treading on. 
It should, however, be immediately observed that if we are 
willing to admit the homology of the amphibian Deckschicht 
with the mammalian trophoblast, we must then unhesitatinely 
go one step further. 
For there is no reason why we should not consider in that 
same light the Deckschicht which we encounter in Ceratodus 
(Semon, 793, ’O01), in Lepidosteus (Dean, ’95), in Acipenser 
(Salensky, ’80, °81), representatives respectively of Dipnoi 
and of Ganoids; nay, we are thus insensibly led to consider 
the Deckschicht of the bony fishes (which is so well known a 
particularity of this class of '‘leleostomes) as also included in 
the group of phenomena about which we are here attempting 
to generalise. 
And it then, of course, strikes us, supposing all these 
different outer covering layers during early larval life to be 
the remnants of an early larval envelope, of which we find no 
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