STUDIES IN SPICULE FORMATION. 243 
time, and finally also deserts. The function of this distal cell 
is that of secreting the thick distal extremity of the monaxon 
before mentioned, and also possibly adding a secondary layer 
of lime to the body of the spicule in the course of its migration 
proximally.! 
It may be added that longitudinal sections of Sycon 
ciliata (Pl. 15, figs. 45—49) show that monaxons may 
originate in the bi-nucleated cytoplasm before the mother- 
cell has separated from the epithelium whence it is 
derived ; however, this is not by any means always the case, 
since many of the youngest monaxons are to be found em- 
bedded in the middle of the wall-substance, and apparently 
not in any way connected with either the dermal or gastral 
epithelium (figs. 48 and 49). Still, monaxons are found thus 
medianly situated, which still retain a connection with the 
gastral wall by means of fine protoplasmic processes, as shown 
in fig. 47. 
THE TRIRADIATE SPICULE. 
Trios of more or less approximated spicule-cells (fig. 13) 
are to be found in every sponge-preparation, and these 
incipient congeries must be regarded as the first observable 
stages in the development of the triradiate (and quadriradiate) 
spicules. That the aggregate of three cells (fig. 14)—the 
“trefoil”? (Minchin)—is formed by the association of three 
cells, and not by the continued division of one, is evidenced 
(apart from other considerations discussed below, which leave 
no room for doubt upon the subject) by the fact that aggre- 
gates of three cells showing two nuclei of smaller size than 
the third are never found, whereas aggregates of four cells 
containing two nuclei and aggregates of five cells containing 
four nuclei, of less diameter than their companions, occur 
1 Since this was written Prof. Minchin has found that such a secondary 
thickening distinctly occurs in the monaxons of Leucosolenia variabilis 
(Ascon). And still more recently I have detected the same in ammonia- 
carmine preparations of Sycon coronata (Pl. 15, figs. 41 and 42), 
