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 fur 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 

 unci bis zu ihrer Erkenntnis hat es noch gute Wege ; das eine kann 

 man aber wohl heute schon sagen, die Keimblatterlehre gehort 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 



