254 W. WOODLAND. 
spicule is deposited—it is possible to consistently explain the 
existence of the three kinds of spicules characteristic of cal- 
careous sponges, showing not only why the three kinds of 
spicule occur, but also why other kinds do not. 
Immigration of dermal cells into the median gelatinous 
substance of the sponge wall mostly occurs in those portions 
of the sponge which are in course of rapid growth, as, e. g., 
in the oscular rim, and in the diverticula of immature sponges. 
As already implied these isolated dermal cells or scleroblasts 
are incapable of depositing lime in appreciable quantity whilst 
in the uni-nucleated condition. Inthe majority of Clathrinidee 
and some other sponges, and also in the very young stages of 
many sponges which possess monaxons at a iater stage of 
development, the stimulus to lime secretion, whatever may be 
its nature, is not even called into existence when the sclero- 
blastic basis of the future spicule is bi-nucleated (and bi- 
nucleated scleroblasts and two-cell associations are to be 
found), but in the majority of Leucosoleniide and Sycons 
monaxon spicules are produced either when the nucleus of a 
single scleroblast divides, or when two scleroblasts associate 
together, or on both occasions (see p. 273). As already indi- 
cated, the bi-nucleated, i.e. two-massed, scleroblastic basis 
uecessarily produces a monaxon structure under such condi- 
tions owing to the elongation of the secreting layer of calco- 
plasm involved in the bi-polar redisposition of the mass of 
the cell-substance, i.e. the monaxon form is directly 
related to the conformation of the secreting agency. 
When three scleroblasts associate together, the conditions 
as regards secretion are somewhat more complex. It is 
evident a priori that a monaxon cannot be formed between 
any two of the three cells, since the presence of the third 
(the potency of which is equal to that of either of the other 
two) must exercise a disturbing influence; in other words, 
three approximately equal secretory centres being present 
and grouped about a common centre, the deposition of 
calcareous matter must be symmetrical with regard to all. 
Why calcareous matter is not deposited at the centre of the 
