PORIFERA 



35 



porous meshwoi'k oontainiiio^ air (air-chamber layer), in 

 which skeletal elements (needles or ampbidiscs) are often 

 found embedded (6), while outside of all there may be added 

 still another cuticular layer (a). Furthermore the germ- 

 body is said to be immediately enveloped in a delicate 

 membrane (Carter). 



The gemmulffi are found in the midst of the mesodermal tissue of the 

 parent body. Many views have been advanced in regard to their origin. 

 According to Goette (No. 6), there is a kind of cell-proliferation that 

 affects a particular territory and also involves the ampullffi and canals of 

 this region, whereas, ac- 

 cording to Maeshall (No. 

 30), certain mesoderniic 

 cells, filled with reserve 

 food-stuff, creep together 

 in groups to form the 

 gemmulfe. The earliest 

 fundament of the gem- 

 mula, which is essentially 

 a mass of cells having 

 embryonic characters, 



soon exhibits a differentia- 

 tion of two layers (Goette, 

 WiERZEJSKi). The central 

 mass is composed of large 

 cells in which yolk par- 

 ticles become embedded in 

 ever-increasing quantities. 

 The cells of the outer 

 layer, according to Goette, 

 become club-shaped, and 

 arrange themselves into a 

 kind of elongated epithe- 

 lium enveloping the central mass. This layer, like the spongioblasts, at 

 first secretes a thick cuticula on the inside (the fundament of the inner 

 cuticular membrane. Fig. 12 c) ; the amphidiscs are then formed in the 

 cells of this layer, whereupon it moves outward in order to secrete, likewise 

 from its inner surface, the outer cuticular membrane, Fig. 12 a (Goette). 

 According to Wierze.jski (No. 39), the amphidiscs are not formed in the 

 layer of cylindrical cells mentioned, but in the surrounding tissue, and 

 only later move into this epithelial layer, in which they become arranged 

 in a definite manner. 



The formation of the gemmulaä takes place principally in the fall in 

 parts of the sponge which generally die after gemmulation has taken 



Fig. 12— Gemmula of Spongilla (Ephydatia) 

 fluviatilia (after Vejdovsky). a, outer cuticular 

 layer; h, amphidisc layer; c, inner cuticular 

 layer ; d, germ-body ; p, pore. 



