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STUDIES IN SPICULE FORMATION. 145 
spicule, in which no “cells” exist, and in which the nuclei 
of the undoubted syncytium bear no definite relation to the 
constituent rays. Moreover, as far as I know, there is no 
authenticated instance of any siliceous spicule arising by the 
fusion of initially-separate parts contained in an individual 
syncytium, though of course it may well be that the hexacti- 
nellid spicules are peculiar in this respect as in others. The 
only ground for believing that the hexact spicule arises by 
the central fusion of six initially-separate rods that I can 
imagine is the general similarity which many hexactinellid 
spicules bear to certain Radiolarian skeletons. In Radio- 
larians these rays are usually separate, at any rate initially, 
and in some cases they become fused centrally. If the mode 
of deposition of hexactinellid rays in the spherical syncytium 
can be shown to be at all similar to that of Radiolarian 
spines in the spherical body, a reason certainly exists for 
supposing that the hexact rays may be deposited initially in 
a separate condition, but there is no other reason for this 
supposition that I know of, and personally I believe the six 
rays of the hexact spicule will be found to emerge from a 
spherical granule. 
Tetractinellida and Monactinellida. 
Generalising the large number of statements! which have 
been made by the various authors enumerated in the biblio- 
1 To avoid burdening the text with a somewhat unnecessary account of 
previous work on the scleroblastic development of siliceous sponge spicules— 
an account of which both Minchin (4) and Maas (24) have provided an outline 
—lI append a full bibliography of the subject, mentioning after each paper the 
type or types of spicule, the scleroblastic development of which is described 
within. I may here add that I have not been able to trace the development 
of every type of spicule contained in the genera mentioned in the Introduc- 
tion, and indeed only in one or two cases would this have been desirable (e. g. 
the development of the rosettes of Esperella), since the development of most 
types is already known. In fact, so far as I am aware, only in Chondrilla 
nucula (Tetractinellida), and in Siphonochalina coriacea, Axinella 
polypoides, Suberites domuncula, Microciona atrasanguinea, 
Cliona sp., and Ectyon ovoides (Monactinellida), among the sponges 
