102 UICIIAHD EVANS. 



of all the importance attaclied to it by Zykoff. The columnar 

 cells during their transference from the inner to the outer 

 side of the external ends of the amphidiscs grow out rather 

 than migrate out. The result is that the spaces between the 

 amphidiscs are partly occupied by the inner moiety of the 

 cells, which moiety, being more or less cut off by the outer 

 ends of the amphidiscs, becomes transformed to the paren- 

 chyma-like substance which occupies that position in the 

 mature gemmule. That this is true can be easily seen on 

 examination of fig. Ibh (PI. 4), where the inner ends of the 

 columnar cells are already undergoing the above-mentioned 

 transformation, though the amphidiscs are not yet in position. 

 Consequently the origin of this layer need no longer be con- 

 sidered unknown, as has been done by Goette and Zykoff. 



The next question to be considered is the origin of the 

 amphidiscs. Lieberkiihn describes the amphidiscs as being- 

 developed in some of the cells of the peripheral layer (see 

 p. 83). (xoette figures a developing amphidisc in one of 

 these cells, and describes these spicules as being formed from 

 within outwards. As has already been pointed out, incom- 

 pletely developed amphidiscs are never seen in the gemmule 

 coat (p. 92), but are abundant in the sponge tissue. They 

 seem to be invariably symmetrical in form, one end being the 

 exact counterpart of the other. Goette seems to have been 

 in error on both these points. 



Zykoff merely confirms Wierzejski's view that the amphi- 

 discs are formed outside the gennnule, but neither of them 

 was able to find the scleroblast, which is most surprising, 

 seeing that Lieberkiihn says that the outlines of the cellular 

 structures containing the amphidiscs are as sharp as those of 

 ordinary sponge cells. Zykoff discusses at considerable length 

 the mode of migration of the amphidiscs from the sponge 

 tissue to the gemmule coat, and arrives at the somewhat 

 amusing conclusion that they are pushed from one position to 

 the other by the sponge cells, much in the same way, I should 

 imagine, as a colony of ants carries away bits of food which 

 are too heavy a bundle for one. The presence of amphidiscs 



