THE GASTR^A THEORY AND ITS SUCCESSORS. 83 



blastula by the invagination of certain of its cells, and 

 represented ancestrally by the hypothetical Gastrcea. 

 This forms the last stage common to all the Metazoa ; 

 from it various paths branch off : '* sie f iihren von der 

 monaxonien Gastrula einerseits zu den monaxonien Spon- 

 gien und den stauraxonien Acalephen, anderseits zu den 

 dipleuren oder bilateralen Bilaterien ; und zwar zunachst 

 zu den Wiirmern, aus denen sich die vier typischen 

 Stamme der Mollusken, Echinodermen, Arthropoden, 

 und Vertebraten erst spater hervorgebildet haben." 



Such is in outline the Gastraea theory, the first attempt 

 to plan out from embryological data the phylogenetic 

 origin of the Metazoa. One of the greatest difficulties 

 in the way of its acceptance is the occurrence of delami- 

 nation ; that is, the conversion of the monoblastic blastula 

 into a diploblastic organism, not by invagination, but by 

 the separation off, by karyokinetic division, of the inner 

 ends of the blastula cells. Some of Haeckel's followers 

 have endeavored to overcome this difficulty by rejecting 

 as improbable Fol's observations on the delamination 

 in Gejyojiia, but renewed study of the development of 

 the Trachymedusae by Metschnikoff and Brooks have in- 

 contestably demonstrated the occurrence of the process. 

 Haeckel, however, met the obstacle more fairly, and, rely- 

 ing on the fact that in comparatively closely related 

 forms both modes of endoderm formation may be found, 

 held the view that delamination is a secondary condition 

 derived from invagination, failing however to explain how 

 it has been derived, and thus leaving the difficulty as 

 great as before. 



Shortly after the publication of the Gastraea theory 

 Ray Lankester brought out his PlanuUi theory, the key- 



