240 THE president's address. 



one end (the future apex), where a distinct knob has made its 

 appearance, and at a point a little above the middle, surrounded 

 by the lower ring of formative cells. At the next stage (fig. 3) 

 the apical knob has grown larger, and a third thickening has 

 appeared in the middle of the upper ring of formative cells. A 

 little later a knob, the commencement of the manubrium, also 

 appears at the other end of the spicule, while each of the two 

 thickenings corresponding to the two rings of formative cells is 

 seen to consist of three conical projections arranged in a whorl, 

 and the entire shaft of the spicule between the knobs and whorls 

 has also become thickened, though to a less extent (fig. 6). I 

 have not been able to detect any formative cells in relation with 

 the terminal knobs, though I think it quite probable that such 

 may be present. 



The coincidence of the positions of the median and subsidiary 

 whorls with those of the rings of formative cells suggests very 

 strongly that the latter are directly concerned in their develop- 

 ment, and the conviction that this is the case becomes irresistible 

 when we study the appearances represented in figs. 4 and 5, 

 Already in the earlier stages (figs. 2 and 3) it had become evident 

 that the formative cells constitute a kind of sheath, partially, 

 if not completely, enveloping the spicule. Around and between 

 the median and subsidiary thickenings this sheath is separated 

 from the spicule by a considerable interval. Presently it bulges 

 out around each of the two thickenings to form three pockets 

 (figs. 4 and 5) which no doubt correspond in position to the 

 three formative cells. It also becomes distended to a less ex- 

 tent between the thickenings and above and below them, and 

 this distention seems to take place only on one side of the shaft, 

 though this lop-sided appearance may possibly be due to post- 

 mortem changes. 



I believe that the distention of the sheath is due to the 

 accumulation within it of a very watery solution of silica — or 

 possibly some silicate. In fig. 4 an accessory silicoblast is seen 

 lying between the two rings of formative cells, and it is sugges- 

 tive that it occurs on the same side as that on which the pockets 

 are beginning to form. In the walls of these pockets traces of 

 the formative cells can still be detected. I imagine that they 



