28 EMBRYOLOGY 



that statement has recently been confirmed by the observations of 

 0. M.^\s {Zool. Anz., 1889), who has convinced himself on the same object 

 that the ectoderm of the larva is not cast off, but gradually passes over 

 into the superficial pavement epithelium of the adult sponge. Also in 

 regard to the origin of the ampullfe and canal system Gaxin does not 

 agree with Goette. According to Ganin, the entodermic cavity is to be 

 explained as an archenteron, and represents the earliest fundament of 

 the canal system, from which the ampuUffi arise as diverticula. A mode 

 of origin for the amiDullae similar to that communicated by Goette for 

 Spongilla has recently been maintained by Dexky (Quart. Juiir. ][licr. 

 Sci., 1888) for one of the horn sponges (Stelospongia). 



The distribution of the recognized types of development among the 

 different groujjs of sponges, then, is represented as follows : In the 

 calcareous sponges (Calcarea) the ami)hiblasiula is found in most of 

 the cases observed up to the present time. Perhaps this larval form 

 is confined to the Calcarea. The coeloblastula appears in Oscarella and 

 in the family of the Plakinidae, whereas the parenchymula seems to be 

 present generally in the Geratosa and siliceous sponges. Furthermore 

 Halisarca and Ascetta exhibit a parenchymula stage. 



As is to be seen from the preceding, a uniform plan of 

 development for sponges cannot at tlie present time be 

 formulated. "^ The statements differ too widely. In certain 

 cases we find a coelogastrula-stage, which attaches itself to 

 some support by the circumference of the large gastrula 

 mouth. This is of importance as a distinguishing charac- 

 teristic in contrast with the Cnidaria, in which the attach- 

 ment is always effected by the aboral pole of the two-layer 

 planula larva. In other cases there is formed a parenchymula, 

 the genesis of Avhich for many forms is as obscure as the 

 further development of this stage into the adult sponge. 

 We can only assume conjectural ly that this stage is in all 

 cases brought about by epibolic g;xetrulation or by the 

 process of migi-ation of certain cells from the entodermic 

 pole. The most obscui*e point in the development of 

 sponges is the metamoi-phosis accomplished at the moment 

 of the attachment of the swarming larva. Even the state- 

 ments regarding the pole by which the larva attaches itself 

 vary for the different forms. Authors likewise differ in 

 regard to the organogeny, above all as regards the origin 

 of the canal system and the flagellate ampullae. In certain 



' Compare footnote, p. 22 [Tkanslatorsj • 



