MATERIALS FOR A MONOGRAPH OF THE ASCONS. 307 
Hach of the two formative cells plays a peculiar and 
characteristic réle in the production of the spicule; one 
places itself more proximally, that is to say, deeper in the 
‘body-wall, and migrates towards the gastral surface; the 
other remains in a more distal, that is to say, superficial 
position, and is found in, or near to, the dermal epithelium. 
The proximal cell is conspicuous as a rule, for its more 
irregular form and amoeboid appearance. As it travels 
inwards it forms the shaft of the spicule, and its chief 
function appears to be to determine the length and direction 
or curvature of the spicule. Like the apical formative cells 
of the rays of the triradiates, its influence is apparently 
chiefly directive; hence it will be briefly referred to in the 
sequel as the founder. On the other hand, the more 
superficially placed formative cell 1s more compact as a rule, 
and is concerned chiefly with building up the spicule to its 
full thickness. Like the basal formative cells of the tri- 
radiates, its function is essentially secretive; hence it will be 
termed for short the thickener. 
Founder and thickener are at first scarcely distinct from 
one another, and the first sign of the spicule is seen between 
the two nuclei, but chiefly in the sphere of influence of the 
thickener (figs. 83—5, 9, 42, 45, 48, 76, 77). The earliest 
part of the spicule to appear is the distal extremity, the end, 
A further point which Woodland has raised, and upon which it is more 
difficult to obtain decisive evidence, is whether or not the monaxons have in 
all cases a single mother-cell. He is of opinion that in some cases two 
distinct epithelial cells co-operate, and furnish the two formative cells. His 
observations could, however, be interpreted as meaning simply that in some 
cases the division of the primary mother-cell is complete, in other cases incom- 
plete; and since the two cells become perfectly distinct later on, as Woodland 
himself has shown, the former procedure is more likely to be the primitive 
one. For my part I am inclined to regard the monaxons as uniform in origin 
and to refer them in all cases to a single mother-cell, but from the nature of 
the case it is almost impossible to give a decisive judgment on this point; 
nor do I consider it a fair argument to attempt to prejudice the point, as one 
might be tempted to do, by the analogy of the formation of the rays of the 
triradiates. 
