Ill 



POEIFERA 



49 



cytes, devoid of yolk, but with plentiful dark granules in their cyto- 

 plasm, which appear to act as carriers of nutriment to the enclosed 

 yolk cells. The investing cells secrete a membrane on their inner 

 surfaces. The columnar layer is then invaded by another class of 

 wandering cells which come from the adjacent tissues of the sponge. 

 These are clear cells carrying in their interior peculiar spicules 

 called amphidiscs. The amphidisc resembles a pair of toothed 

 wheels joined by an axle. Various stages in the development of 

 amphidiscs can be seen in the maternal tissues. They first appear 

 as little needles, similar in shape to the other spicules of the sponge. 

 The ends of the needles thicken and eventually form wheel-like discs 



'TV x%&%w%?\ 

 start X qS%tC 



FIG. 26. Section through a gemmule-bearing individual of Ephydatia Uembingia. 



(After Evans. ) 

 amph, Amphidisc ; gemm, gemmule. 



(Fig. 26 ampTi). When the amphidiscs have taken up their position 

 amongst the cells of the investing layer, the cells which carried them 

 degenerate and disappear. 



Before the investment of columnar cells is quite complete, the 

 trophocytes withdraw from the yolk ^ cells and pass back into 

 the mother sponge. The inner ends of the columnar cells, after 

 having secreted the membrane, likewise degenerate and form a sort 

 of network of fibres between adjacent amphidiscs. The outer portions 

 of these cells, however, become segregated off from their inner de- 

 generating portions and pass back into the mother sponge. Before 

 doing so, however, they secrete on their inner ends another membrane, 

 which may be called the outer membrane of the gemmule, since it 

 unites together the outer ends of the amphidiscs. Where the investing 



VOL. I E 



