34 EMBRYOLOGY 



ner the budding in Tethja, Tetilla, Rinalda, etc. (Dfiso, No. 

 29; Mekejkowsky, No.31; Selenka, No.34), and alsoinLopho- 

 caljxi. e. Polylophus (F. E. Schulze, No. 33), appears to de- 

 pend npon the outgrowth and abstriction of a portion of the 

 parent body, in which is included a part of tlie canal system 

 of the latter, while the tissues exhibit an active cell- 

 proliferation. The separation from the parent frequently 

 takes place in these cases by an emigration along pi"ojecting 

 siliceous spicules (Fig. 11). After separation has taken place, 

 the bud grows up into a young animal resembling the parent 

 organism. To this class belong also the transportable brood- 

 buds of Oscarella described by F. E. Schclze (No. 32), which 

 are very similar in structure to the lar^a of this form (Fig. G 

 D), since they contain a considerable internal cavity. These 

 vesicular bodies, after they have become separated from the 

 parent, are for a time carried about by currents, and then 

 fall to the bottom, whei'e they grow into young sponge- 

 crusts. 



Reproduction by budding in these forms depends upon the 

 fact that the superficially located parts of the sponge tissue 

 become separated and acquire the power of I'eproducing the 

 entire form of the parent organism. If we consider a simi- 

 lar process to take place within the tissues of the sponge, 

 whereby the separation of such a group of cells is accom- 

 plished by an encystment, then perhaps the manner is indi- 

 cated by which we are to consider the fii^st formation of 

 gemmula; to have taken place. Reproduction by means of 

 gemmulae occurs principally among the fresh-water sponges 

 (Spongilla), although the occurrence of gemmula-like 

 structures has also been affirmed for a few marine forms 

 (Topsent, No. 35). The fully developed gemmula (Fig. 12) 

 consists of a multicellular germ (rf), whose large cells, poly- 

 gonally flattened by mutual pressure, are filled with yolk 

 particles, and present one, frequently two or more, nuclei 

 (Petr, Weltner). This germ-body is sux'rounded by an 

 envelope often very complicated in structure, which opens to 

 the exterior by means of a pore (p) provided with an oper- 

 cular apparatus. There is always found a thick cuticular 

 layer (c), to which there is generally adiled externally a 



