EMBRYOLOGICAL CRITERION OF HOMOLOGY. 107 



made for possible processes of substitution). Ectoderm and 

 entoderm are the original bases of all tissues and organs (with 

 the probable exception of the germ-cells) in the Coelenterata : 

 the same holds true of the ectoderm and entoderm of embry- 

 onic forms." * Justification for this implicit trust in the primary 

 germ-layers as a fixed and absolute standard of comparison was 

 sought, on the one hand, in the supposed constancy of their 

 relation to corresponding adult parts (ectoblast always giving 

 rise to nervous structures, entoblast to the digestive epithelium, 

 etc.), on the other hand, in their alleged homology to the layers of 

 the diblastic ancestral type (that " mageres Thiergespenst " the 

 Gastraea of Haeckel, etc.) from which all other forms have 

 descended. 



The final steps in this direction were the attempts to deter- 

 mine the origin of parts even in the pregastrular or cleavage 

 stages by tracing out the cell-lineage or cytogeny i.e., the 

 derivation of particular parts from individual blastomeres of 

 the segmenting ova. This method, which for a time seemed to 

 give brilliant promise, took as a starting-point the unsegmented 

 ovum ; and thus exhausted the possibilities of observation. 



It is plain from the foregoing analysis that the phrase 

 "similarity of embryological origin" is used with great lati- 

 tude, sometimes denoting merely a similar relation to well- 

 differentiated larval or embryonic parts (e.g., parts derived 

 from the skeleton of the first visceral arch); sometimes a simi- 

 lar mode of origin with direct regard to the germ-layers 

 (ventral nerve-cord of annelids and arthropods) ; sometimes a 

 similar relation to the cleavage-stages (products of the neuro- 

 blasts in leeches and chaetopods). 



It may be shown, in the first place, that not one of these 

 stages can in itself be taken as a fixed standard of homology. 

 Let us first consider the larval and embryonic stages, which 

 may be conveniently considered together. It is a familiar 

 fact that parts which closely agree in the adult, and are un- 

 doubtedly homologous, often differ widely in larval or embry- 

 onic origin either in mode of formation or in position, or in 

 both. Innumerable cases will suggest themselves to any em- 



1 Kleinenberg: Lopadorhynchus, p. 18. 



