THE PRINCIPLES OF EMBRYOLOGY 46 1 



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 decidiug 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 Gastraea ; 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 ; 

 but 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 Brasm, is signifi- 

 cantly absent in the earliest Metazoa ; in those very places where, on 

 the Gastraea 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 alin*sntary 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) 



