80 A. A. W. HUBRECH'. 
We can now understand how the particular mode of 
amnion development as we find it in birds, and more especially 
in the chick (which was naturally the type upon which all 
speculations concerning the amnion were originally based, 
no other being sufficiently known) has given rise to that 
erroneous conception of the amnion as the first cause of the 
production of the serous membrane. The error was all the 
more explicable, but at the same time all the more tenacious 
because in the chick the existence of the trophoblast as an 
extra larval envelope is not in any way evident (see p. 20). 
It is only by placing together all the transition stages from 
the more primitive mammals to the Ornithodelphia and the 
Sauropsida, that we can succeed in demonstrating how it comes 
about that the ectodermal layer of cells which in the latter 
gradually travels round the yolk and finally encloses it is 
not primarily the radial extension of the ectoderm, sensu 
strictiori, of the shield, but that it owes its origin to what 
we have learnt to distinguish as an enveloping layer, which 
has lost its significance in the oviparous ‘sauropsids, and can 
only be seen in its full detail in mammals. 
A yet further reduction, or, to put it more correctly, a 
reduction in yet another direction than what we notice in 
Sauropsida is presented by the trophoblast of the Amphibia. 
Not in all, but in very many of these the development is 
characterised by the fact that at a very early period the 
outer ectodermic layer of the young embryo is so visibly 
different from the subjacent ectodermal cell-layers that it is 
distinguished by the name of ‘‘ Deckschicht ” from the latter 
which are termed the *‘ Grundschicht.” Moreover, the cells of 
the Deckschicht proclaim their transitory and larval signifi- 
cance yet further by the fact that they disappear in later 
developmental stages, and that it is only into the constitution 
of peculiar larval organs that they play any part. So in 
that of the sucking disc, and in that of what are considered 
as larval olfactory organs (ig. 131). 
This then is a decidedly transitory, we may even say larval 
layer. Ona former occasion (’95) I have already committed 
