328 WILSON. [Vou. 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. 70 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, Pl. XX, 
Fig. 69 g. 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, sf., 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. sz., 
Figs. 70 and 73. It is probably formed much as the corre- 
