152 W. WOODLAND. 
the spicule as in the other two groups, but merely adhere to 
the spicule-ray as cylinders or semi-cylinders. ‘These differ- 
ences as regards the method of spicule formation in the 
three groups of sponges doubtless possess phylogenetic 
significance. 
SuMMARY. 
1. The earliest stages in development of the Hexactinellid 
spicule are at present unknown, but there exist reasons for 
supposing that it originates as a granule enclosed by a 
spherical syncytium in which cell-outlines are absent, and 
that the six rays of the hexact grow out from this granule. 
2. The earliest stage of development yet discovered is the 
small hexact, the rays of which do not extend to the periphery 
of the enveloping syncytium. 
3. The rays of the hexact elongate, causing the spherical 
syncytium at first “to present a more or less octahedral shape, 
with somewhat concave surfaces and with rounded corners ” 
(Ijima). 
4, The rays at length extend beyond the spherical contour 
of the syncytium (the scleroblast mass), the scleroplasm of 
which, however, persistently adheres to the rays as a thin 
film, which occasionally includes nuclei. 
5. The peripheral growth of the megasclere type of spicule 
causes the spherical syncytium enveloping the point of junc- 
tion of the six rays to dwindle and finally to disappear on 
account of the distension involved. The whole of the micro- 
sclere, with the exception of the terminals, remains per- 
manently enveloped by the spherical syncytium. 
6. All tetractinellid and monactinellid spicules originate as 
granules contained within single cells. In a few instances 
the spicule arises from several granules within the cell and 
then consists of separate parts (dragmata, for example). 
7. All growth is accretionary. There is no well-authenti- 
cated instance of a siliceous sponge-spicule being formed by 
