STUDIES IN SPICULE FORMATION. 297 
in its mode of fission—the mass of the cytoplasm becoming 
concentrated at two centres (each with its nucleus), and 
therefore attenuated in the region situated between these,— 
and lime salts being deposited most freely where the bulk of 
the cytoplasm is greatest, the sclerite must obviously, under 
these conditions, not only, like the cytoplasm, become elon- 
gated, but also, like the cytoplasm, become dumb-bell shaped, 
as we know it does (text-fig. A). So far, then, the Alcyonarian 
spicule follows the normal course of development. But, as 
before stated, beyond the dumb-bell stage of growth, the 
position of the two nuclei does not seem to exert any influence 
on the form gradually assumed by the spicule — processes 
being developed on all sides quite irrespective of the two 
TEXT-FIG. A. 
thickenings of the general protoplasmic investment containing 
these bodies,—and the further morphogenesis appears to be 
solely related to external factors. In what manner, however, 
environmental conditions produce the various forms charac- 
teristic of adult Alcyonarian spicules is a question somewhat 
difficult to answer, and at present I can only supply a sug- 
gestion or two towards the solution of this problem. 
Two general principles at once present themselves for 
consideration, the first of which is that growing spicules 
situated in a mass of mesoglceal substance far removed from 
any limiting surface must, owing to the proximity of other 
spicules, endodermal canals, and other heterogeneities of con- 
stitution of the surrounding medium, necessarily be subject 
to an aggregate of influences which tend to produce irregu- 
larity of form ; and the second is that the extension of a grow- 
ing body into a surrounding resistant medium is most easily 
effected by the protrusion of more or less acute processes 
