320 W. WOODLAND. 
of the skeletal area marked out by the presence of sclero- 
blasts, i.e. in the course in which further extension of the 
arms of the original triradiates will occur. These isolated 
deposits, which mostly originate in the pseudopodial processes 
connecting the cells of the blastoporic ring, are shown in 
figs. 8,12, 25. The rod in fig. 25 has doubtless originated 
between two cells once closely apposed but which have since 
separated—the apposition of two cells being, at least in 
calcareous sponges, an essential condition to the production 
of an elongated structure. In fig. 25 also the triradiate, in 
all probability, arose in a clump of cells—at least three— 
which have since dispersed to join the adjacent longitudinal 
strand. It is probable that these independent deposits largely 
contribute to the small processes and other irregularities 
characterising the adult larval skeleton. 
‘he three rays of each of the two triradiate spicules once 
produced gradually elongate in their three respective paths. 
The curved transversely-disposed rays (the “ recurrent apo- 
physes”’) ultimately meet ventrally in the median plane, but 
are not in the majority of cases, contiguous in the same line 
(figs. 27. 28) ; the rays pointing anteriorly—away from the 
blastopore—extend upwards to form the supports of the an- 
tero-dorsal arms; the rays pointing downwards, or towards 
the blastopore, extend posteriorly for a short distance and 
then curve suddenly dorsally (tending to thus complete dor- 
sally the calcification of the blastoporic ring already accom- 
plished ventrally by the recurrent apophyses) to form later 
the “ baguettes de corps ” or main terminally-thickened pos- 
terior supports of the larva; thus the blastopore becomes 
(relatively) shifted to the ventral side. At the point where 
these postero-dorsal arms of the original triradiate bend dor- 
sally a pair of new arms originate to form the supports of a 
pair of postero-ventral arms. 
The projection of a new pair of arms from any point of the 
previously formed skeleton is always preceded at that point 
by a small cluster of scleroblasts. As Théel says, “ As a rule, 
calciferous cells are present at such parts of the increasing 
