STUDIES IN SPICULE FORMATION. 143 
siderably in size, and this feature is probably related to the 
different kinds of spicules to be produced. So few really 
young stages exist in my material that I am quite unable to 
say of what types of spicule my figures represent the young 
stages ; possibly it would be difficult to say under any circum- 
stances, since the young hexact must be an initial develop- 
mental stage common to all types, both megascleres and 
microscleres. 
Further, although I am unable to demonstrate it by actual 
examples,’ I may say that I think it is highly probable that 
the initial granule from which in all probability the 
six rays of the hexactin grow out, is found within a 
Syncytium, and not, as all other siliceous sponge 
spicules are,in a single cell. In support of this state- 
ment I may adduce the following evidence: (a) cell-clusters 
similar to those represented in fig. 1, are occasionally found 
in the tissues of the sponge, and are apparently identical 
with those enveloping young spicules; (l) the large number 
of nuclei present in the syncytium containing the young 
spicules figured confirms the view that more than one nucleus 
was present at the origin of the spicule ; and (c) this distinc- 
tion between the modes of origin of hexactinellid and other 
siliceous sponge spicules is only one of many distinctions 
which separate the Hexactinellida as a group from other 
siliceous sponges. 
The great extension of the rays of the megasclere type of 
spicule during the later stages of development in every case 
causes the centrally-situated syncytial mass to decrease in 
size and in some cases to vanish, since the scleroplasm is 
needed for peripheral growth. Ijima suggests that, since in 
calcareous sponge spicules the scleroblasts ultimately desert 
the spicule, the syncytium of the hexactinellid spicule pos- 
sibly does the same, and, as is well known, numerous other 
authors have often asserted that this desertion does actually 
1 T searched with the greatest care for a syncytium containing a siliceous 
granule, spherical or six-cornered, but, as stated in the text, I met with no 
well-defined stage younger than that figured. 
