PORIFERA 



25 



ampullae and all of the canals arise as evaginations of this inner cavity. 

 Later the osculum breaks through, and siliceous spicules are formed in 

 the cells of the mesodermic layer (W. Maeshall, No. 10). 



Halisarca. — By total and equal cleavage (F. E. Schulze, No. 20) there 

 originates a blastula, into which migrate cells that entirely fill the 

 cleavage cavity, and there form a connective-tissue mesenchyma. Upon 

 emerging the larva presents at the jDOsterior pole an area consisting of large 

 granular flagellate cells. After the larva has attached itself and assumed 

 a cake-like form, the ectoderm loses the flagella, and is changed into a 

 pavement epithelium. In the internal parenchyma there now arise 

 fundaments of the ampullar and canals which at first are separate, but 

 subsequently unite into a common system (Metschnikoff, No. 1-4). 



A unique type of development, which perhajis most 

 resembles Reniera, appears to exist in Spongilla. The de- 



Fig. 8. — Late stage of cleavage (beginning gastrulation) of Spongilla (Epkydatia) 

 fiuviatilis (after Goette). Ec, ectoderm cells; En, entoderm cells; d, central 

 entodermlc cavity. 



velopment of this fresh- water sponge has been described by 

 Ganin (Nos. 4 and 5) and Goetie (No. 6) ; but in many 

 points the statements of these investigators do not altogether 

 agree. In our presentation we follow^ the detailed descrip- 

 tion of Goette without forming any preliminary judgment 

 as to the way in which the development of Spongilla is 

 related to that of other sponges. A tinal judgment will be 

 possible only when new observations have been made on 

 the development of sponges of the most widely diffei'ing 

 groups. 



The egg of Spongilla (Ephydatia) fiuviatilis undergoes 



