258 W. WOODLAND. 
actinoblast in the same degree that an isolated scleroblast is 
able to, but, as is shown by the basal cells of all young 
triradiates, the mere presence of a cell is sufficient to stimu- 
late another to active work (the one in the meantime remain- 
ing passive so far as secretion is concerned), and hence the 
basal cells of the triradiate can well fulfil this condition in 
the formation of a quadriradiate spicule. 
But this last assumption naturally suggests a further 
question. Jf an unspecialised cell of the internal oscular 
epithelium is able, when brought into proximity with the 
basal cells of a triradiate spicule, to forcibly compel these to 
co-operate (Lorcibly, since a change or lapse of function is 
induced) and share in the production of a monaxon spicule, 
how is it that isolated scleroblasts do not by similar means 
produce adventitious rays on the opposite side of the tri- 
radiates’ Isolated scleroblasts must often come into the 
vicinity of triradiate actinoblasts, and hence, on the above 
assumption, might have been expected to initiate deposition 
under such circumstances. ‘I'he answer to this question is 
afforded by the constancy with which the apposition 
of the two cells is maintained in the case of the 
gastral ray. ‘This constancy of immediate apposition 
obviously results from the disposition of the triradiate with 
regard to the gastral wall in which the future gastral 
actinoblast is situated, and is evidently not present in the 
case of an isolated moving scleroblast situated on the dermal 
side of the triradiate. In the former case persistent main- 
tenance of the apposition forcibly induces the co-operation 
of the triradiate basal cells; in the latter case the conditions 
do not permit of such coercion, and in this distinction doubt- 
less lies the explanation of the difference of results in the 
two cases. 
‘his forced co-operation between dermal cells, one or other 
(or both) of which is previously engaged in another function, 
is still more notably illustrated by the induced division of a 
pore cell, situated in the body-region of the sponge (below 
the oscular rim), to provide the gastral actinoblast for a 
