THE EMBRYOLOGY OF THE PORIFERA. 



IN few groups of the animal kingdom has the true 

 nature of the embryonic development been so little 

 understood, or the statements of the investigators of this 

 subject so contradictory, as in sponges. An attempt to 

 reduce their ontogeny to a common type was made by 

 Haeckel, who wished to fit them in with his Gastraea 

 theory, and under the influence of this idea regarded all 

 sponge larvae as gastrulae ; in fact he even went so far as 

 to depict the larvae of Ascetta primordialis and other 

 species as free swimming gastrulae of a most typical kind. 1 

 It was soon shown, however, by the more accurate studies 

 of Schmidt, MetschnikofT and Schulze on the embryology 

 of Calcarea that Haeckel's figures and descriptions were 

 far from being true to nature, and that the larvae of A scons 

 could in no way be regarded as gastrulae, while the process 

 of invagination undergone by the Sycon larvae was in some 

 respects the reverse of a typical gastrulation. 



By the careful investigations of Schulze and Mets- 

 chnikofT on the development of Sycandra, the ontogeny 

 of this form became better known than that of any other 

 sponge, and a distinct type of development was established, 

 characterised by an amphiblastiila larva, composed half of 

 columnar ciliated cells, half of granular, rounded, non- 

 ciliated cells. This larva was found to be characteristic 

 of Sycons and Leucons generally, and also of Ascandra 

 among Ascons. It is thus more or less universal among 

 Calcarea, with the exception of the most primitive forms 

 [Ascetld). The most striking feature of this type of 

 development is that the ciliated cells become invaginated 

 into, or overgrown by, the granular non-ciliated cells, 

 and that the former cells give rise to the collar-cell layer 



1 Die Kalkschwamme, 1872 ; compare also Natiirliche Schopfungsge- 

 schichte, fifth edition, Berlin, 1874, pp. 454 et seq., taf. xvi. 



