318 E. A. MINCHIN. 
they were faintly stained. One would then get the impres- 
sion that the entire spicule was the product of one cell, 
namely the gastral actinoblast. I may point out that so 
careful an observer as Metschnikoff made, many years ago, a 
similar misinterpretation of the sextett surrounding the 
young triradiates. 
It is never safe to be dogmatic about things one has not 
seen, but I venture, nevertheless, to express my belief that it 
will be found to be universally true that the triradiates of 
calcareous sponges develop from six cells,! the quadriradiates 
from seven. 
As regards the subsequent development of the triradiate 
systems, the process of events is quite similar to what I - 
formerly described in Clathrina, as may be seen from my 
illustrations. The first deposits of calcareous matter are 
very irregular (figs. 19—21), but soon take definite shape as 
a symmetrical spicule, and, concomitantly, the cells of the 
sextett sort themselves out into three pairs of formative cells, 
two attached to each ray in a definite and uniform manner. 
One formative cell, placed more towards the gastral aspect of 
the body-wall, behaves in all respects like the “ founder” of 
the monaxon spicule ; the other, placed more on the dermal 
side, 1s the “ thickener” (figs. 24—28, 52, 58, 92—95). The 
founder is to be sought for at the tip of the ray, the thickener 
at the base; hence these two cells were distinguished by me 
in a former memoir as the apical and basal formative cells, 
respectively. As development proceeds, there comes a period 
when the founder is no longer to be observed, having wan- 
dered off from the ray. Itis with regard to this point that I 
wish to correct some of my former statements. In Clath- 
rina I figured a quite young spicule, in which the rays were 
far short of their full length, without any founder-cells 
attached to the extremities of two of the rays (1898, pl. 38, 
fig. 9). I am now convinced that this condition was simply 
due to the founders having been brushed off, since they are 
1 That is to say, in the first instance from three mother-cells, each of which 
divides into two, thus giving rise to the sextett. 
