THE PRINCIPLES OF EMBRYOLOGY 461 
the germinal layers, and shows how they were originally a purely 
physiological conception, and how gradually such conception changed 
into a morphological one, with the result that what had up to that 
time been looked upon as analogous structures became strictly homo- 
logous and of fundamental importance in deciding the position of any 
animal in the whole animal series. 
This change of opinion was especially due to the lively imagina- 
tion of Haeckel, who taught that the germinal layers of all Metazoa 
‘must be strictly homologous, because they were all derived from a 
common ancestral stock, represented by a hypothetical animal to 
which he gave the name Gastreea; an animal which was formed by 
the simple invagination of a part of the blastula, thus giving rise 
to the original hypoblast and epiblast, and he taught that throughout 
the animal kingdom the germinal layers were formed by such an. 
invagination of a part of the blastula to form a simple gastrula. If 
further investigation had borne out Haeckel’s idea, if therefore the 
hypoblast was in all cases formed as the invagination of a part of 
a single-layered blastula, then indeed the dogma of the homology of 
the germinal layers would be on so firm a foundation that no specula- 
tion which ran counter to it could be expected to receive acceptance ; 
hut that is just what has not taken place. The formation of the 
gastrula by simple invagination of the single-layered blastula is the 
exception, not the rule, and, as pointed out by Braem, is signifi- 
cantly absent in the earliest Metazoa; in those very places where, on 
the Gastrea theory, it ought to be most conspicuous. 
Braem discusses the question most ably, and shows again and 
again that in every case the true criterion upon which it is decided 
whether certain cells are hypoblastic or not is not morphological but 
physiological. The decision does not rest upon the answer to the © 
question, Are these cells in reality the invaginated cells of a single- 
celled blastula? but to the question, Do these cells ultimately form 
the definitive alimentary canal? The decision is always based on 
the function of the cells, not on their morphological position. Not 
only in Braem’s paper, but elsewhere, we see that in recent years the 
physiological criterion is becoming more and more accepted by 
morphologists. Thus Graham Kerr, in his paper on the development 
of Lepidosiren, says: “It seems to me quite impossible to define a 
layer as hypoblastic except by asking one or other of the two ques- 
tions: (1) Does it form the lining of an archenteric cavity ? and (2) 
