EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 27 
to which the name of trophoblast has been given. This layer, 
phylogenetically subordinated to the ectoderm, was looked 
upon as a differentiation of the same order as the outer larval 
layer which in certain Nemertines, Gephyreans, and other 
worms often serves as a temporary envelope that is stripped 
off when the animal attains to a certain stage of development. 
In a later chapter it will be discussed whether the different 
foetal envelopes of the Amniota allantoidea may not be brought 
into genetic relation with this layer, and whether we might 
be justified in thus tracing the foetal envelopes of the higher 
vertebrates as far back as the invertebrate ancestors provided 
with an ectodermal larval investment (Larvenhiille). 
It would appear at first sight probable that in the Anamnia, 
Anallantoidea (i.e. in the Ichthyopsids) traces of this larval 
cell-layer should not be met with, and that this very absence 
would help to explain the fact that here no amnion develops, 
However, the chance that the intrinsic differences between 
say Amphibia and Reptiles are not so incisive as this separa- 
tion of the vertebrates in Amniota and Anamnia would make 
us believe, should also yet receive our consideration. And it 
is in this light that I intend to look upon the fact that in. many 
amphibia certain ontogenetic stages reveal the presence of 
what has been called the ‘“‘Deckschicht” of the larva. 
Numerous figures successively published by different authors 
show the extent to which such a layer has been actually 
observed. It should at the same time be noticed that in 
several other genera no trace of it has been found. 
The more remarkable circumstance is, however, this—that 
not only in Amphibia such a ‘‘ Deckschicht ” makes its occa- 
sional appearance, but that similarly it is noticed during the 
development of certain Dipnoi and Ganoids (Fig. 87), and both 
more constantly and more unquestionably during that of all 
the Teleosts (Fig. 89) of which up to now the early develop- 
ment has been traced. Of these different groups the “ Deck- 
schicht ” is here figured after the publications of different 
authors on the subject, and I will not here enter into further 
details, contenting myself with having shown that it is a 
