DESCRIPTION OF EPHYPATIA BI,EMR[N(![A. P3 



mately grows to the hat-shaped disc. Throughout the pro- 

 cess of foniiation of the amphidisc both ends are of the 

 same shape. If one end is incompletely developed, the other 

 is equally so. The spicules retain their position inside the 

 scleroblast until they have reached their definitive form, and 

 there seems to be no reason for supposing that, were the 

 scleroblast in any way injured, the spicule could ever attain 

 full development. The amphidiscs thus described assume 

 their ultimate form while yet in the general tissues of the 

 sponge. It is important to remember that they are developed 

 in cells which are essentially amoeboid. When gemmules are 

 being developed, the scleroblasts in virtue of their inherent 

 poAver of locomotion move towards them. They travel along 

 the strands of tissue which have been described above as 

 passing from the general sponge tissue to the somewhat loose 

 membrane which surrounds the gemmule. Ultimately they 

 make their way among the columnar cells which suri-ound 

 the gemmule (PI. 4, figs, 14 and 156). Fig. 156 (PI, 4) is 

 particularly interesting in that it shows the last spicule that 

 has entered the columnar layer as well as one situated in a 

 strand of tissue close by. The latter is on its way to take up 

 its position alongside the former spicule among the colunniar 

 cells. When the amphidiscs, still situated inside the sclero- 

 blasts, have reached their final ])osition, at first they are 

 longer than the columnar cells, which lie completely inside 

 their outer ends. At this stage the scleroblasts, thouo-h 

 already considei'ably modified, can be distinctly seen. In the 

 fully grown gemmule, however, they are indistinguishable 

 from the parenchyma-like substance produced from the modi- 

 fied inner ends of the columnar cells. 



The scleroblasts with their contained amphidiscs first push 

 their way in among the columnar cells at that point where 

 the columnar layer and the inner chitinous coat made their 

 first appearance. They become more numerous and gradually 

 increase in number until finally they envelop the whole 

 genniiule (PI. 4, figs. 15 and 16). There are, therefore, three 

 distinct structures iit least which first appear on the same 



