460 THE ORIGIN OF VERTEBRATES 
between these arises a third layer, the mesoblast. These layers are 
strictly morphological conceptions, and are stated to be homologous 
in all cases, so that the hypoblast of one animal must be homologous 
to the hypoblast of another, In order, therefore, to compare two adult 
animals for the purpose of finding kinship between them, it is neces- 
sary to find whether parts such as the gut, which in both cases have 
the same function, arise from the same germinal layer in the embryo. 
We can, in fact, have no certainty of kinship, even although the two 
animals are built up as far as the adult state is concerned on a 
remarkably similar plan, unless we can study their respective 
embryos and find out what parts arise from the hypoblast and what 
from the epiblast. The homology of the germinal layers constitutes 
in all cases of disputed relationship the court of final appeal. A 
new gut, therefore, in any animal can only be formed from hypoblast, 
and any theory, such as that advocated in this book, which deals 
with the formation of a new gut, and does not form that gut from 
pre-existing hypoblast, must of necessity be wrong and needs no 
further consideration. 
Such is the result of current conceptions—conceptions which to 
be valid must be based upon-an absolutely clear morphological 
definition of the formation of the germinal layers, a definition not 
based on their’ subsequent history and function, but determined solely 
by the uniformity of the manner of their origin. 
What, then, is a germinal layer? How can we identify it when it 
first arises? What is the morphological criterion by which hypoblast 
can be distinguished from epiblast, or mesoblast from either ? 
This is the question put by Braem, in an admirable series of 
articles in the Biologisches Centralblatt, and is one that must be 
answered by every worker who bases his views of the process of evolu- 
tion upon embryological investigation. As Braem points out, the 
germinal layers are definable either from a morphological or physio- 
logical standpoint. In the one case they must arise throughout on 
the same plan, and whatever be their fate in the adult, they must form 
at an early stage structures strictly homologous in all animals. In 
the other case the criterion is based on function, and the hypoblast, 
for instance, is that layer which is found afterwards to form the defi- 
nitive alimentary canal. There is no longer any morphological homo- 
logy ; such layers are analogous ; they may be, but are not necessarily, 
homologous. Braem gives a sketch of the history of the views held on 
