THE GASTR^A THEORY AND ITS SUCCESSORS. 89 



terate development have felt strongly the insufficiency 

 of these theories to explain the phenomena occurring 

 in the early stages of the ontogeny of forms belonging 

 to that group, and have been led either to throw doubt 

 upon their applicability to these cases (Brooks, von 

 Lendenfeld, Goette), or have exhibited considerable 

 intellectual elasticity in endeavoring to bring about a 

 harmony. Let us glance at the methods of formation 

 of the diploblastic embryo in the lower Metazoa, before 

 passing on to a consideration of the next theory. 



In Sponges there seems little room for doubt that 

 the solid embryo or planula, as we may call it, is of much 

 more frequent occurrence than the invaginate gastrula. 

 In Ascetta a blastula results from segmentation, and by 

 the migration of cells situated at one pole of this struc- 

 ture, a solid central mass of cells is produced, and a 

 similar process (perhaps assisted by delamination) prob- 

 ably occurs in Halisarca, Reniera, Esperia, and other 

 forms ; at all events, there is nothing in the formation of 

 the central cells of the embryos of these forms which 

 indicates the occurrence of invagination. This process 

 is exceptional in the Sponges, and so far as is known 

 at present occurs only in some of the simpler calcaregus 

 sponges and in Oscarella, and in these cases presents 

 some peculiar characteristics which throw doubt upon 

 the homology of the Gastrula of these forms with that 

 of such a form as Sagitta. 



In the Cnidaria the absence of an invaginate gastrula 

 is quite as striking as in the Sponges. It is unknown 

 in the Hydrozoa, in which migration of cells of the 

 blastula resulting in the formation of a solid planula 

 may be regarded as the rule, though in the Trachyme- 



