MATERIALS FOR A MONOGRAPH OF THE ASCONS. 351 
Fic. 28.—Older quadriradiate. The gastral actinoblast, with two nuclei, 
has a polygonal outline with incurved sides and sharp corners, caused by 
fitting in between the bases of the collar-cells. 
Fic. 29.—A nearly full-grown quadriradiate with all six formative cells. 
Gastral actinoblast not drawn. 
Fies. 830—36.—Derelict triradiates and quadriradiates, some with a few 
cells still attached, others with none. All occurring close to the dermal 
surface. 
Figs, 87—41.—Histology and spicule-formation of Leucoso- 
lenia complicata, as seen in sections. 
Fic, 37.—An ordinary collar-cell (on the left) and two sister-cells resulting 
from recent division. In the latter the collars are not yet formed, and the 
flagella are short, thickened at their base, and tapering to a point. 
Fic. 38.—A cell of the dermal epithelium, mushroom-like type; the body 
of the cell, containing the nucleus, is placed far down the surface, and sends 
out delicate processes. 
Fre. 39.—A cell of the dermal epithelium in which the nucleus has recently 
divided. 
Fic. 40.—Two sister-cells of the dermal epithelium resulting from recent 
division. 
Fig. 41.—Cells of the dermal epithelium passing in to form a spicule 
(compare Figs. 1 and 2). 
PLATE 18. 
Histology and spicule-formation of Leucosolenia complicata as seen in 
sections (collar-cells drawn in outline, except in Fig. 53). 
Fic. 42.—Section showing dermal epithelium and a young monaxon spicule 
(m.sp.) with its two scleroblasts. No nuclei of epithelial cells come into the 
section. The other parts of the section are not figured. 
Fig. 43.—Section showing a young monaxon spicule (w.sp.) with its two 
- formative cells (the distal end of the spicule broken off); two cells of the 
dermal epithelium (d.ep.); five collar-cells, and amongst them an excretory 
cell (e2.c.). 
Fic. 44,—Section showing a monaxon spicule (m.sp.) at a late’ stage of 
growth (compare Fig. 7); the spicule is much broken in cutting. Two cells 
of the dermal epithelium and three collar-cells are also seen. 
Fic. 45.—Portion of a section through the region where the sponge is 
attached to an alga (4.); the dermal epithelium (d.ep.) becomes at this point 
very columnar; the mesoglcea is greatly thickened. sc/.¢. Scleroblast attached 
