PORIFERA 



35 



porous meshwork containing air (air-chamber layer), in 

 which skeletal elements (needles or ampliidiscs) 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 gemmulse 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 

 aiJects a particular territory and also involves the ampullas and canals of 

 this region, whereas, ac- 

 cording to Marshall (No. ^ 

 30), certain mesodermic 

 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, 

 WiEEZEJSKi). 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 ei^ithe- 

 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 Wieezejski (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 gemmulse takes place principally in the fall in 

 parts of the sponge which generally die after gemmulation has taken 



Fig. 13. — Gemmula of Spongilla (Ephydatia) 

 fluviatilis (after Vbjdovskt). o, outer cuticular 

 layer ; b, amphidisc layer ; c, inner cuticular 

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



