MATERIALS FOR A MONOGRAPH OF THE ASCONS. 305 
now rely on the region of the oscular rim, where the collar- 
cells are entirely wanting. By carefully cutting off the 
sponge-wall in this region and laying it out flat without any 
further treatment, stages of spicule-formation can be found 
and drawn, aud thus any possible error arising from manipu- 
lation is entirely eliminated. 
In Leucosolenia the dermal epithelium covering both 
aspects of the oscnlar rim is comparatively free from the 
coarse granulations found in Clathrina, a fact which makes 
the former genus a particularly favourable object for demon- 
strating the stages of spicule-formation, even to classes of 
students. It is interesting to find that in these days of 
advanced technique, there are still histological facts to be 
discovered by methods which might. have been feasible to 
Lieberkiihn or Max Schulze. 
The investigations set forth in the following pages are not 
quite so complete in some points as I could have wished. 
Since, however, I now hold an appointment in which it is 
accounted to me for unrighteousness to look at a sponge 
(except for domestic purposes), | am obliged to put forward 
my results as they are without spending time upon further 
investigations. 
TI]. Tar Monaxon SPIcULeEs. 
The formation of the monaxon spicules of Leucosolenia 
takes place in a manner essentially uniform, in all cases 
observed by me, whatever the shape or size of the spicule. 
The process may therefore be described in general terms, 
-and the variations observed in particular cases will be briefly 
noted afterwards. As a preliminary it should be pointed out 
that the full-grown monaxon spicules of Leucosolenia (the 
statement can, perhaps, be extended to the primary 
monaxons of all Calcarea) are placed invariably with one 
end, which may be called proximal, imbedded in the body- 
wall, while the opposite, or distal extremity, projects freely 
into the water. Haeckel (1872, vol. i) states that the 
