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 Biologisclies 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 



