STUDIES IN SPICULE FORMATION. 559 
Fic. 11.—The formation of the young plate containing four perforations. 
The number of the cells ranges from two to five. 
Fie. 12.—Further growth of the young plate; eight cells present, 
Fics. 13—16.—Other forms of the (4) variety of spicule in C. sp., all 
biscleroblastic. 
Fic. 17.—Knobbed spicule-plate (a) with two scleroblasts. 
Fies. 18, 19.—Knobbed spicule-plates with four scleroblasts. 
PLATE 33. 
Fies. 20—30.—Stages in development of the (c) variety of spicule in C. 
sp. Mostly, if not all, biscleroblastic in origin. Fig. 22, however, is probably 
tetrascleroblastic. Figs. 25—30 x about 375. 
Fics. 31—86.—Young stages in the development of the plate-spicules of 
C. brunnea. In fig. 85 the elongated celi to the left probably does not 
belong to the spicule. 
Fies. 37, 38.—Elongated cells—‘ fibres”—and the green cells found in 
great numbers in the body-wall of both C. sp. and C. brunnea. 
Fie. 39.—Stages in the development of the superficial spicules common to 
both C. sp. and C. brunnea. ¢, e, and / are stages found, which, if perfect, 
prove the one-celled origin of this form of Cucumarian spicule. 
Fics. 40, 41 show the adult form of superficial spicule, with either one or 
two scleroblasts attached. 
Fie. 42 shows another form of adult superficial spicule, with four sclero- 
blasts. 
Figs. 43, 44 show the final stage of scleroblastie development of the plate- 
and-anchor spicules of Synapta inhaerens. Fig. 44 shows the spicule in 
side view. Both figures x about 375. 
PLATE 34. 
Stages in development of the spicules of Thyone fusus. 
Fics. 45—56 represent a developmental series of the stool spicules. 
Figs. 48 and 54 probably represent spicules of tetrascleroblastic origin. 
Fig. 55 represents a stool spicule in side view. 
Figs. 57—65 represent a similar developmental series of the superficial 
spieules. In Fig. 60, one end of the spicule is apparently broken. 
