612 E. A. MINOHIN. 



persists long enough to show that it plays an important part 

 in the growth of the spicule. Its disappearance is, indeed, a 

 very remarkable fact, taking place as it usually does at a time 

 when the sister formative cell is still at or near the base of the 

 ray, and in all cases so far removed from the apex that a con- 

 siderable stretch of the ray is left exposed and, to all appear- 

 ance, covered only by the sheath. 



So far as the further growth of the spicule is concerned, the 

 apical cell no longer exists for us, but the question as to what 

 becomes of it is of considerable interest. Unfortunately this 

 question is one very difficult to answer positively. When the 

 apical cell disappears, it is not to be supposed that it has any 

 other fate than that of becoming merged in some class of 

 cells, amongst which it is, or rapidly becomes, indistinguish- 

 able in its characters and appearance. It is scarcely possible 

 that it should become a collar-cell or a wandering cell, both 

 on general grounds and because it is sufficiently distinct in all 

 its characters from cells of either class. It is also highly 

 improbable for the latter reason that it should become a pore 

 cell, for which office its inferior size alone would disqualify it, 

 so to speak. If all these possibilities are excluded, there 

 remain only two classes of cells into the ranks of which it 

 could be received; namely, the flat epithelium and the con- 

 nective-tissue layer. It is by no means impossible that after 

 leaving the apex of the spicule ray it should become an actino- 

 blast, and join with other cells like itself to form a spicule. 

 All the actinoblasts, however, which I have seen were exces- 

 sively granular, while the apical cells have few or no large 

 granules, since those which they originally possessed became 

 absorbed, as we have seen, during the growth of the ray, I 

 have seen no appearances which suggest that the actinoblasts 

 originate from the cast-off formative cells of spicules already 

 formed, but many which indicate an origin for them from the 

 flat epithelium by immigration. If, however, the actinoblasts 

 come from the epithelium, there seems no great difficulty in 

 supposing that their daughters, the formative cells, return 

 there. The one feature of the latter which at all marks theuj 



