366 WILSON. [Vol. VI. 



larval form — and this, as I believe and shall endeavor to show 

 further on, is an unwarrantable assumption which ultimately 

 leads to the most contradictory and perplexing results. That 

 this is really Kleinenberg's meaning is, I think, proved by his 

 reasoning throughout the entire paper. It is strikingly illus- 

 trated, for example, in his treatment of the annelid trochophore 

 (p. 176). After pointing out certain so-called homologies be- 

 tween the organs of the trochophore (ring-nerve, ring-muscle, 

 ciliated belt) and those of a medusa (ring-nerve, ring-muscle, 

 velum or umbrella-margin), he says : — 



" Giebt man diese Homologien zu, so wiirde man ein fertiges 

 geschlechtsreifes Thier von der Organisation der Lopado- 

 rhynchuslarve im System gewiss entweder in die Ordnung der 

 Hydromedusen einstellen oder wenigstens am nachsten zu 

 diesen setzen. Die Principien der Klassifikation miissen aber 

 dieselben bleiben, ob es sich um die Endstadien oder um die 

 Zwischenstadien eines Evolutionscyclus handelt, und wenn ich 

 in der Entwicklung eines Annelids eine Form finde, die von der 

 Annelidenorganisation gar nichts besitzt, dagegen in den wesent- 

 lichsten Theilen einer Meduse gleichartig ist, so nehme ich sie 

 auch nicht fiir ein Annelid sondern fur eine Medusa." 



In passing I may remark that such " principles of classifica- 

 tion " lead to a complete reductio ad absiirditin when applied to 

 the earlier stages of development (see p. 441). Here, however, 

 I call especial attention to the fact that the passage is devoid of 

 meaning, either in itself or in connection with those already 

 quoted, except under the fundamental assumption that the inner 

 and outer layers of the larva (and of the gastrula) are respec- 

 tively homologous with those of the coelenterate body. 



It appears, therefore, broadly speaking, that Balfour and 

 Kleinenberg reason in precisely opposite directions, the prem- 

 ises of either being the conclusions of the other. Balfour, assum- 

 ing the homology of the mesoblast (with certain reservations 

 that do not affect the general result) is logically compelled 

 to deny the precise homology of the ectoblast and entoblast. 

 Kleinenberg, on the other hand, assumes the homology of the 

 primary layers, and is then led over the same path, but in the 

 reverse direction, to the conclusion that the mesoblast is not 

 homologous. Kleinenberg takes the gastrula as a primary or 

 ancestral larval form ; while Balfour, if I understand him cor- 



