PORIFERA 25 



k 



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

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



Hallsarca. — 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 posterior 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 ampullae and canals which at first are separate, but 

 subsequently unite into a common system (Metschnikoff, No. 14). 



A unique type of development, which perhaps most 

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



En 



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

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

 entodermic cavity. 



velopment of this fresh- water sponge has been described by 

 Ganin (Nos. 4 and 5) and Goette (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 final judgment will be 

 possible only when new observations have been made on 

 the development of sponges of the most widely differing 

 groups. 



The egg of Spongilla (Ephydatia) fluviatilis undergoes 



