514 E. A. MINOHIN. 



spicule. Comparing figs. 11 and 14 with figs. 9 aud 10, one 

 striking point brought out is the great activity of the originally 

 basal formative cell. The apical cell usually disappears at so 

 early a period that by far the greater bulk of the spicule ray 

 must be secreted by the basal cell alone. Further, a com- 

 parison of different stages makes it evident that the spicule 

 ray grows in length as well as in thickness after the apical cell 

 has gone, and while the remaining formative cell is still at the 

 base. This is conclusive evidence that the ray is enveloped 

 completely by a layer of protoplasm even when the formative 

 cell is quite at the base and the apex is apparently exposed, 

 although the refraction of the spicule makes it impossible to 

 distinguish clearly any such enveloping layer of protoplasm 

 except near the nucleus. 



During the growth of the ray the formative cell undergoes 

 certain changes. First^ the granules, which, as we have seen, 

 have all along been diminishing in number and size, continue 

 this process until they are at last completely absorbed. At the 

 same time the protoplasm also appears to diminish slightly in 

 quantity during the period of most active secretion, probably 

 on account of its being spread over the whole spicule ray ; as 

 a consequence, the nucleus becomes slightly compressed be- 

 tween the spicule and the surface of the cell, and has an oval 

 outline in side view and a circular outline in surface view. 

 As the result of these changes the condition is attained which 

 is characteristic of all fully formed spicules — a clear finely 

 granular cell, with a slightly compressed nucleus, adherent to 

 the extreme tip of each spicule ray. 



The above account of the formation of the spicules is 

 based, as has been said, upon their development in coriacea, 

 but in the other species studied by me the process of spicule 

 growth is so similar in all essential features that I have no 

 hesitation in regarding it as the normal and typical mode 

 of development for the triradiate systems of the genera 

 Clathrina and Ascandra. 



The dififerences in the development of the spicules in difiFerent 

 species affect only matters of detail, and are largely such as are 



