STUDIES IN SPICULE FORMATION. 249 
figures of the gastral rays given, this must be borne in mind. 
The gastral ray attains a considerable size before its mother- 
cell divides into two (figs. 39 and 40)—at least two thirds of 
the adult length. Division occurs about midway up the 
length of the ray, and the two cells so produced apparently 
tend to separate. The further destiny of the two cells I have 
been unable to trace, probably because no further develop- 
ments occur, the two cells persistently adhering to the ray 
throughout the life of the sponge. 
As implied, never more than two cells are associated with 
the development of the gastral ray in S. coronata and 8. 
ciliata, but, as in Ascons, it is probable, nay certain, that 
this limit of cell divisibility is merely a specific feature, not 
necessarily holding for other species and genera in which the 
spicule attains larger dimensions. In fact, a comparative 
study of calcareous spicules shows beyond doubt that the 
number of formative cells produced from the original mother- 
cell (or mother-cells) is, other things equal, strictly correlated 
with the maximum size of the spicule attained (from the 
standpoint of cause and effect, the order should obviously be 
reversed), and hence in the case of exceptionally large struc- 
tures the number of actinoblasts is above the average. 
One feature in which the formation of the gastral ray 
differs from the type of development presented by the 
monaxons and triradiates is the apparent production of the 
ray in a single cell, the second not being formed until avery 
advanced stage of growth, and then bearing a very different 
relation to the spicule compared with that of the basal cell of 
triradiates or the distal of monaxons; that is, the second 
gastral actinoblast is concerned with the further growth 
of the ray, and not with its initial production. As 
pointed out below, it is probable 4 priori that one or more 
of the basal cells of the triradiate base co-operates with the 
division product of the pore-cell in order to supply the initial 
stimulus necessary, under normal conditions of growth, to 
the production of a monaxon structure, 
