Manchester Memoirs, VoL I. (1906), No. 3. 69 



chen,' and Hubrecht a ' Stomodaeum,' looking for the 

 primitive Coelenterate gut in the cavity of the blastocoel. 

 But, perhaps, it will be maintained, this is a mere caenoge- 

 netic adaptation, and the embryo does possess in this 

 stage to all intents and purposes a primitive gut opening 

 by a blastopore, an ontogenetic type of frequent, though 

 not — as witness the Planula theory — of universal occur- 

 rence. The desired homologies are now sustained only 

 by explaining away as caenogenetic every inconvenient 

 fact, which is suicidal ; by refusing to define a cavity by 

 the nature of its walls, which is absurd ; and by ignoring 

 dissimilarities in origin, though origin is in every other case 

 held to be the sole ultimate test of community of descent, 

 which is inconsistent. Such reasoning must surely be the 

 very last refuge of the destitute phylogenist. 



In fine, if we establish homologies, as we are perfectly 

 entitled to do, on the basis of the structure which the ^^^ 

 presents at the end of segmentation these homologies are 

 without constant reference to those resemblances which 

 appear in the next stage during the closure of the blasto- 

 pore ; and these, in their turn, bear no necessary and 

 definite relation to the structure of the adult forms. 

 Either origin or destiny may be taken as decisive of 

 homology ; but origin and destiny do not of necessity 

 coincide. The appeal to destiny must either rob the 

 supposed recapitulatory stage of all independent signi- 

 ficance or involve us in a vicious circle, while any attempt 

 to make origin the ultimate criterion will inevitably end 

 in the subversion of that homology which we are bound 

 to predicate of the adult organs through and through. 

 The Vertebrata then afford no support to the Gastraea or 

 to any other theory which attaches a morphological, 

 phylogenetic, or recapitulatory significance, a value for the 

 determination of adult homologies, to the germinal layers. 



