462 THE ORIGIN OF VERTEBRATES 
Does it become a certain part of the definitive epithelial lining of 
the gut?” 
The appearance of Braem’s paper was followed by a criticism from 
the pen of Samassa, who agrees largely with Braem, but thinks that 
he presses the physiological argument too far. He considers that 
morphological laws must exist for the individual development as well 
as for the phylogenetic, and finishes his article with the following 
sentence, a sentence in which it appears to me he expresses what is 
fast becoming the prevailing view: “ Mit dem Satz, den man mitunter 
lesen kann: ‘es muss doch auch fiir die Ontogenie allgemeine Ge- 
setze geben’ kann leicht Missbrauch getrieben werden ; diese allge- 
meinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand 
und bis zu ihrer Erkenntnis hat es noch gute Wege; das eine kann 
man aber wohl heute ae sagen, die Keimblatterlehre gehért zu 
diesen allgemeinen Gesetzen nicht.” 
I conclude, then, that we ought to go back to a time previous to 
that of Haeckel and ask ourselves seriously the question, When we 
lay stress on the germinal layers and speak of this or that organ arising 
from this or that germinal layer, are we thereby adding anything to 
the knowledge that we already possess from the study of the anatomy 
and physiology of the adult body? If by hypoblast we only mean 
the internal surface or alimentary canal and its glands, etc., and by 
epiblast we mean the external surface or skin and its glands, etc., 
while mesoblast indicates the middle structures between the other 
two, then I fail to see what advantages we obtain by using Greek 
terms to express in the embryo what we express in English in the 
adult. 
The evidence given by Braem, and it could be strengthened con- 
siderably, is conclusive against the morphological importance of the 
theory of the germinal layers, and transfers the fundamental impor- 
tance of the early embryonic formation, from that of a three-layered 
embryo to that of a single-layered embryo—the blastula—from which, 
in various ways, the adult animal has arisen. 
The derivation of both arthropod and vertebrate from such a 
single-layered animal is perfectly conceivable, even though the gut of 
the latter is not homologous with the gut of the former. We have 
seen that the teachings of embryology, as far as its later stages are 
concerned, afford one of the main supports upon which this theory 
rests. What, therefore, is required to complete the story is the way 
