510 E. A. MINCHIN. 



shows it for CI. coriacea; PI. 39, figs. 18, 19, and 21, for 

 Clathrina, sp. dub., figs. 23 and 24 for embryos of falcata, 

 and fig. 27 for embryos of cerebrum; PI. 40, fig. 33, for 

 reticulum; PI. 41, fig. 38, for contorta. In coriacea when 

 the rays of the spicule have attained a length of about 20 /a, 

 each ray has on it two cells, one at the base and one at the 

 apex. The basal cell appears as a more or less fusiform mass, 

 applied to the spicule ray, and containing the nucleus in 

 its thickest part. The surface of the cell is usually smooth 

 and rounded, without processes. The apical cell, on the other 

 hand, has a very irregular outline, and runs out into processes 

 in a way which gives the impression of its being amoeboid and 

 motile. It is placed quite at the tip of the spicule ray, its 

 nucleus lying on a level with the point or even beyond it, and 

 the portion of the cell body in contact with the ray is relatively 

 very small. All tlie formative cells have now become much 

 denser through gradual absorption of the granules with which 

 at an earlier stage they were packed, and the diminution and 

 disappearance of the granules goes on pari passu with the 

 growth of the spicule. The apical formative cells appear rather 

 clearer and less granular than the basal cells, which is perhaps 

 partly due to their being more spread out and less compact. 



The next event in the history of the spicule is the dis- 

 appearance of the apical formative cells. The period at which 

 this occurs is somewhat variable, not only in the case of dif- 

 ferent species, but in the spicules of the same individual, and 

 even for the rays of the same spicule. In ray preliminary 

 account I stated for CI. coriacea that the apical formative 

 cells, which I then wrongly identified with the outer cells of 

 the sextet, disappeared when the spicule rays had attained a 

 length of 10 ju or 15 fx. More extended observations have 

 shown me that this is far too low an estimate for the average 

 length of ray at the time when this event takes place. Al- 

 though the apical cells are often not to be found at this early- 

 stage, they more frequently persist until the spicule ray has 

 attained a length of at least 20 /i, and sometimes even to a 

 much later stage of growth. 



