SPONGES 



77 



iion-ciliated dermal layer (Fig. 60,/), the cells of which may form 

 precocious spicules, so that both the metamorphosis and the histo- 

 genesis may be said to begin before fixation and during the larval 



Fig. 00. 



Developiiient of Si/coti raphamis (after Schulze). o, dviini ; b, c, ovum segmentiiii,'— ?) as sepii 

 from above, c, as seen from the side ; il, blastosphere with eifrhtC/) posterior gnmular cells 

 (archaeocytes),(listiiii;iiisli(>(l by their darker ajipeaiance; c, free swimmiiif; larva (aniphiblastula); 

 the more centrally placed archaeocytes are not seen ; /, later stage of the same, showing the 

 ciliated cells becuming overgrown by the non-ciliateil ; a, optical section of pupa in which the 

 gastral cavity has appeared ; note the two rounded cells, evidently porocytes, bordering on the 

 cavity; h, j, young si)onge (0/v"'/i».<) showing the newiy-fornied osculum with an iris-like 

 contractile membrane from which the oscular rim is formed ; h in side view, j, seen from above. 



period. If we compare this larva witli that of Lencosohnia, as de- 

 scribed above, we see that it differs from it only in the fact that 

 the germ layer formation is thrown back, so to speak, from the 

 larval to the embryonic period, so that the Sycon amphiblastula 



