32S WILSO.X. [\'0L. IX. 



which I preserved, in spite of the great abundance of full-sized 

 gemmules, it was a very difficult matter to find medium sized 

 and quite small ones. The only explanation I can think of is, 

 that after a certain period no more gemmules are produced in 

 the sponge, but those already formed are allowed to mature 

 and develop into the swimming larva. In this way it might 

 come about that in the latter part of the season a sponge 

 should contain only mature gemmules, and I suppose it was 

 towards the end of the season when my material was preserved. 

 A few stages in the formation of the gemmule were, however, 

 observed. A small gemmule is shown in Fig. yo g. Its shape 

 and the fact that it lies free in one of the canals indicate that 

 it is amoeboid and was creeping about when killed. In struct- 

 ure it is precisely like the ripe gemmule, consisting of a finely 

 and uniformly granular mass, the granules taking a deep stain. 

 No cell boundaries nor nuclei were visible. Some very small 

 masses, consisting of the same finely granular material and in 

 which again no nuclei nor cell boundaries could be made out, 

 were occasionally found imbedded in the mesoderm, PI. XX, 

 F'o- ^9 S- These, from their histological similarity to the 

 older stages, were construed as very young stages in the 

 formation of gemmules. Now it will be remembered that in 

 the mature gemmule of Esperella the cells are so closely 

 packed and are so full of fine yolk granules, that the cell 

 boundaries are very indistinct and the appearance is given to 

 the gemmule of a uniformly granular mass with nuclei scattered 

 through it, the nuclei being so small as to look like mere 

 chromatin masses. This is what I suppose to be the true 

 structure of the Tedania gemmule. I take it to be a mass of 

 mesoderm cells in which the cell boundaries, owing to the 

 compression of the cells and abundance of yolk, and the nuclei, 

 owing to their small size, are obscured. 



In the mature gemmule a few spicules or pieces of spicule 

 are usually found. Fig. 70, sp., which undoubtedly have not 

 been formed in the gemmule itself, but have got in from the 

 maternal tissue. The sheath or capsule which surrounds the 

 gemmule is made up of closely packed fibre-like cells, g. sh.. 

 Figs. 70 and 73. It is probably formed much as the corre- 



