THE president's ADDRESS. 239 



arrangement of the silica, and (3) the accessory silicoblasts, which 

 supply the formative cells with the necessary material. 



I may remark in passing that this hypothesis may help us 

 to reconcile the apparently conflicting observations of certain 

 authors who have dealt with this subject. Thus my friend Dr. 

 Woodland, in the last of his " Studies in Spicule Formation," * 

 contradicts the statements of Maas as to the development of the 

 anisochela in Esperella. Maas says that four nuclei occur in 

 connection with each spicule. Woodland says that there is 

 only one. They are dealing with distinct species, it is true, but 

 it is unlikely that there is any specific difference in this respect, 

 and it seems at least possible that Woodland has seen only the 

 large accessory silicoblast while Maas has seen the formative 

 cells. Similarly I am now inclined to interpret the single, large, 

 so-called " nucleus " figured by myself in connection with the 

 anisochela of Cladorhiza inversa,'\ as an accessory silicoblast rather 

 than as the nucleus of a mother-cell. 



An analogy to the supposed behaviour of the accessory silico- 

 blasts is perhaps to be found in the activities of the " nurse-cells " 

 which I described in my memoir on the " Gametogenesis of 

 Grantia compressa.'^ | These cells collect food-material, in the 

 form of other cells, and convey it to the growing ova, just as the 

 accessory silicoblasts are supposed to collect silica and convey it 

 to the formative cells. The silica must, of course, be obtained 

 ultimately from the water flowing through the canal-system, 

 but how this is effected is entirely unknown. It seems probable 

 that an accessory silicoblast is only temporarily associated with 

 a developing spicule, and that, after parting with its load of 

 silica, it moves off again to some favourable locality where it 

 can collect a fresh supply. 



After this somewhat lengthy and speculative digression we 

 may continue our account of the development of the discorhabds. 

 In the stage represented in fig. 2 {Latrunculia hocagei) the deposi- 

 tion of silica upon the protorhabd has already commenced, and 

 thickening has taken place especially at two points, viz. at 



* Quarterly Journal of Microscopical Science, N.S., vol. 52. 

 t Report on the Challenger Monaxonida, Plate XXI. fig. 13. 

 X Quarterly Journal of Microscopical Science, N.S., vol. 60. 



