STUDIES IN SPICULE FORMATION. 293 
presently to be described), and is, in fact, the starting point 
of the morphological differentiation of the spicules. In form 
it somewhat resembles, as Bourne suggests, a caudal vertebra, 
and like all preceding and succeeding stages is enclosed in a 
granular protoplasmic sheath containing two nuclei, which, in 
the vast majority of cases, are situated at the two extremities 
of the dumb-bell basis. 
It is an interesting fact that in all succeeding stages of 
spicule-formation—in all the varied and complicated forms 
which adult spicules assaume—only two nuclei are present. 
It is important to insist upon this point since Bourne states 
that “in older and more complicated spicules I have [he has] 
counted three or four nuclei.” Personally, in all the hundreds 
of spicules which I have observed I have only once, possibly 
twice, chanced upon one possessing more than two nuclei; in 
one case the spicule was at the “caudal vertebra” stage of 
development, and of the ordinary form, although perhaps with 
some extra processes developed, and possessed as many as six 
nuclei, all apparently contained within the protoplasmic in- 
vestment of the spicule ; in another case the spicule possibly 
possessed four nuclei, although of this I am not so certain. 
In both of these cases I may of course have been mistaken— 
may have misjudged adjacent free scleroblasts as cells apper- 
taining to the spicule,—but in the former example, which I 
most carefully observed and drew, I do not think I was; in 
any case the rarity of such multinucleated spicules quite dis- 
proves Bourne’s statement that such are of common occurrence. 
The fact that Bourne mistook endoderm cells for scleroblasts } 
—‘“the scleroblasts are often coenocytes containing two, three, 
or more nuclei” —was probably the source of his erroneous 
assumption on this head, despite the admission that “the 
nucleus apparently divides when the sclerite has attained 
a certain size,’ and the erroneous statement that “this 
division is repeated as growth [of the spicule] continues.”? I 
may here point out in this connection that von Koch also 
1 It must not be inferred from these little criticisms that I fail to appre- 
ciate the high value of Bourne’s excellent paper. 
