46 A. A. W. HUBRECHT. 
entoderm is clearly indicated as forming part of the ring that 
is figured for Sorex in Fig. 60. 
The ultimate discussion upon this matter is postponed to a 
later publication, in which those stages of development which 
are inaugurated by the formation of the somites will be 
treated more fully. 
IJ. AMPHIBIA. 
After this description of the early developmental processes 
of the Mammalia we will skip the Sauropsida, and first 
describe what is noticed in the Amphibia. ‘This will after- 
wards afford us an occasion to compare the yolk-laden Saurop- 
sida all the more rapidly both ways. And, above all, it will 
increase our confidence in the interpretation which we have 
founded on the Mammaliaif we find it applicable as low down 
in the line of vertebrate descent as are the present Amphibia. 
It should, however, at the same time, be remembered that 
none of the three living stems of Amphibia neither the Gym- 
nophiones, nor the Urodeles, nor the Anura can be expected 
to stand in any way on the direct line of descent of our 
present mammals. Comparative anatomy has taught us 
(Fiirbringer ’00) that in very many respects the amphibious 
Promammalia of the Paleeozoic epoch must have been charac- 
terised by important points of difference from all the living 
remnants of that ancient stem. Still, if we find processes of 
early development that are in the main lines directly com- 
parable to what we have described in mammals, and if they 
fit in well with the explanation which we have ventured to 
give for the Mammalia, we might say that the difficulties 
which have so often been complained of (p. 13) when attempt- 
ing to establish the comparative ontogeny of the Vertebrates 
have greatly diminished. 
We will, therefore, take the more important and careful 
descriptions of Amphibian development (to which we have 
no personal investigations of our own to add), and see whether 
the three centres of proliferation which we have noticed in 
