26 Dr. W. Siilcusky on lliickeVs Gatitri\3U Theory. 



lamellfB constitute a form which resembles the Planula-ionn 

 of the Ca'lenterata and ditfers from this form only in certain 

 .inimals by the further ditierentiation of tlic middle <,^erni- 

 lamella. The other form, from which the ditierentiation first 

 commences, and which eonseciuentiy is not to be compared to 

 the Planula, 1 have called "■ Jildstida,''' merely in order to 

 indicate by this name that developmental state of some 

 animals startinj^ from which ditierentiation of the gcrm- 

 lamellaj occurs somewhat otherwise than in the Planula. We 

 have met with this form in various jinimals, and briefly 

 explained their further ])rocess of ditierentiation. The sim- 

 plest ditierentiation consists in that some cells of the Jilastula 

 be^in to distinguish themselves from the rest. By this, two 

 germ-lamelhc are at once indicated, and the grade of organi- 

 zation equivalent to that of the Planula is attained. The two 

 germ-lamelli\3 may be further developed in difierent ways : 

 either they may constitute a body, which is the Gastrula (as in 

 Amp/iioxus, the Ascidia, «S:c.) ; or the inner germ-lamella may 

 be covered by the outer one, by which no Gastrula-ioxm. is pro- 

 duced (as in the Insecta), 



In these brief remarks on the Gastrma theory I have only 

 desired to bring together the facts with which I endeavoured 

 to clear up the significance of that theory for myself. The 

 negative result at which I arrived rests upon facts, especially on 

 these — that the Gastrula is not of general occun'ence, and that 

 the embryological phenomena cannot be brought into causal 

 connexion with this fundamental form. Even if the Gastrula 

 were of as general occurrence as Hiickel states, this would by 

 uo means prove that it is truly an ontogenetic fundamental 

 form ; for what do we gain by the assumption that the 

 Gastrula is a fundamental form of the development of all 

 I^Ietazoa, if we cannot by this form explain the differences 

 in the development of nearly allied animals (e. g. Amphioxus 

 and other Vertebrata, Ctenobranchiata and the other Proso- 

 brauchiata, &c.) ? i^y the Gastrcea theory we cannot explain 

 the difference in the development of Lumhricus and Euaxes. 

 But very many such examples exist ; and they show that, 

 between animals standing near each other systematically, 

 essential differences may occur in the foundation of their 

 organs. This fact, however, appears so paradoxical only 

 because we are now accustomed to deduce the relationship of 

 animals only from anatomical facts, and to conclude from 

 similarity of organization that there is similarity of develop- 

 mental processes. But in order to ascertain the mutual rela- 

 tions of organized forms, we should employ all the methods 

 of natural history ; we must regard the structure of the 

 mature organic forms as the result of the ontogenetic pro- 



