of the Sponge Organism. 405 



which the various tissue-elements are gradually differentiated 

 from the principal mass of the larva which remains after 

 deducting" the collar-cells. First are separated the supporting 

 skeletal substance and the cell material from which the genital 

 products arise. At a later period the epithelial covering-layer 

 and the contractile elements first become separated. Much later 

 still the cells appear differentiated which glue the spicules 

 together into bundles by secretion of spongin. The ontogeny 

 of Esperia furnishes a good indication as to how these 

 must have developed in tlie course of ontogeny. Naturally 

 displacements and abbreviations in point of time must not be 

 left out of consideration ; thus, for example, in the phylo- 

 genetic history of sponges the formation of spicules and 

 fixation have universally and with reason been brought into 

 connexion with one another ; but a whole series of free- 

 swimming sponge-larva3 show spicules already present. On 

 the whole, however, the sequence of events among themselves 

 and the nature and manner of differentiation may be taken as 

 good guides to conclusions. 



Tlie development of Esperia^ and indeed of Sycandra also, 

 has a parallel in the phylogenetic stage represented by 

 Ascetta clathrus in the sponge series. In this simple sponge 

 we have, according to Minciiin, little more than skeleton and 

 genital products in the intermediate tissue ; the seat of con- 

 tractility lies still especially in the epithelial covering, just 

 as must have been the case, according to the development of 

 Esperia and Sycandra^ in phylogeny, and in very primitive 

 forms must still be. 



From a series of cases in whicli the development of spongin 

 is more accurately known to us we are well justified in 

 speaking of a diploblastic embryo. We could distinguish in 

 it, according to the cases before us, an ectomesoderm and 

 an endoderra ; but these names at once entail a comparison 

 with the germinal layers of higher animals, and the preceding 

 discoveries were intended to be kept within the limits of the 

 group of sponges. 



Berlin, 

 July 20, 1892. 



