242 W. WOODLAND. 
In fig. 3a two cells have apparently associated to otherwise 
produce the bi-nucleated condition of the calcoplasm, and if, 
as seems likely, a monaxon spicule similarly results from this 
somewhat differently-constituted bi-nucleated mass, this 
spicule should exhibit some difference in external appearance 
from one produced in the manner above described. In 
Appendix B I have supplied some evidence in favour of the 
supposition that the thicker variety of monaxons is derived 
from the association of two mother-cells (whose nuclei do not 
undergo division), and the thin from the division of the single 
mother-cell. 
In all monaxons, even the thinnest, the two extremities are 
dissimilar, the proximal end, or end embedded in the sponge 
substance, tapering very gradually to the point, and the 
distal end, or end protruded through the sponge-wall to the 
exterior, being thicker, or, in other words, terminating more 
abruptly, and, in very young forms, resembling a barb (figs. 
5 and 6). ‘This latter feature is doubtless correlated with 
the prolonged adherence of the apical cell now to be des- 
cribed. 
In all early stages of monaxons, the two cells associated 
with the spicule are situated at its extremities (figs. 5, 6, and 
7), but as growth proceeds, the distal cell, after remaining 
stationary for some time, migrates towards the proximal end 
(figs. 8—11) until, in the adult structure, it replaces the 
proximal cell in position, this latter having previously 
deserted the spicule, after constructing the greater portion 
of it (fig. 12). ‘lhe fact that the proximal cell does take 
considerably the larger share in the formation of the spicule 
is evidenced by the constant absence of the distal cell 
on the part of the spicule exposed to the outer world 
(at least two-thirds of the entire length), this evidently 
implying that during the protrusion of the spicule through 
the body-wall, the work of secretion in lengthening the 
monaxon has been solely performed by a proximal agency, 
The originally-distal cell, having replaced the proximal cell, 
adheres to the proximal extremity of the spicule for some 
