AND THE LOWER ANIMALS. 159 



We may regard the reproduction of the Aurelia 

 as typical of the group, although in regard to the 

 number of the phenomena which it exhibits, it holds 

 a median rank. There are far more complex and 

 far more simple forms, but characters of plus or 

 minus do not' alter the real nature of the process of 

 development. 



Without inquiring into their precise signification, we 

 may now indicate two important facts which bear upon 

 those which precede them : firstly, each egg of the 

 Medusa, instead of producing a single fertile animal, 

 as is the case with that of the butterfly, gives rise to 

 several individuals; and secondly, this production 

 takes place mediately ; for between every two genera- 

 tions of Aurelia many races of dissimilar beings are 

 produced by gemmation. 



To speak in a more general manner, we have 

 here the production of several generations, through the 

 medium of a single germ. This is, to my mind, the 

 fundamental fact which rules all the secondary pheno- 

 mena. It is this which I have endeavoured to convey 

 in the term geneagenesis, which is applicable to all forms 

 of reproduction that possess this essential character. 



If with the foregoing history of the Aurelia we 



parasitic Trematodes, rejects Van Beneden's nomenclature, and 

 employs the words embryo, nurse, and larva. In this particular he 

 agrees with the editor of the " Bibliotheque de Geneve," who 

 deprecates the change I have made in Steenstrup's nomenclature. 

 We shall see further on, that as this nomenclature conveyed ideas 

 which I considered inexact, and was out of keeping with the view 

 I took of the entire phenomenon, it was necessary to alter it. 

 Moreover, in adopting the expressions proposed by Van Beneden, 

 I think I have once more shown how little anxious I am to intro- 

 duce new terms into science. 



