316 W. WOODLAND. 
a position situated just above the junction of the strand with 
the blastoporic ring as a centre, the cells are arranged in 
groups diverging from this centre in three directions—one 
group (the strand) extending longitudinally upwards and 
away from the blastopore, and two series of cells stretching 
round the hypoblastis invagination on both sides. Hence it 
follows merely from this general disposition of the cells that 
further growth of the initial deposit must occur more or less 
in these three directions, i.e. in the directions which the 
three rays of the triradiate actually take. In other words, 
the form of the triradiate, viewed from a general standpoint, 
undoubtedly, though roughly, does correspond to the disposi- 
tion of the scleroblasts.1_ However, there is another possible 
factor which must be considered. It is easily observable that 
in the growth of the four spicule rays (two belonging 
to each triradiate) which extend some distance round the 
region of the blastopore, the actual deposition occurs in 
the course of the long protoplasmic processes connecting 
the adjacent cells (figs. 12, 20, 25, 26), or, in other words, 
the deposition takes place in a mould already formed for 
it. Since this is the case for this distal portion of the 
skeleton, it is therefore possible that the moulding takes 
its origin from the cell which contains the initial granule, 
i.e. centrally, and that therefore the initial triradiate form 
is immediately due to this. It is certainly possible on 
occasion to trace a faint streak directly continuous through 
the mass of cells from the tip of the ray of the young 
triradiate to the protoplasmic “cord”? on which the cells 
of the blastoporic ring are “strung,” and therefore it would 
seem that the protoplasm in the course of the growing 
spicule is modified in connection with its further extension. 
At the same time, it happens on occasion that several 
1 In the figures supplied by Selenka (17) of the young triradiate spicules of 
E. miliaris only three or four cells are shown on each side of the gastrula. 
If a longitudinal strand is in this case composed of such a small number of 
cells it is extremely remarkable; but, merely judging from Selenka’s other 
figures, and from the appearauce presented by the adult skeleton in plutei 
which I possess of this species, I doubt if this be the case. 
