236 THE president's address. 



mother-cell which secretes the silica. About the actual exis- 

 tence of these two constituents of the spicule — the protorhabd 

 and its coating of silica — there can be no doubt, but their mode 

 of origin is a much more difficult question, upon which I hope 

 that the following observations may throw some light. 



It is extremely rarely, if ever, that the protorhabd has been 

 detected as an independent structure, before the commencement 

 of secondary thickening by the deposition of silica. In both 

 our species of Latrunculia, however, we find something at any 

 rate very closely approaching to this earliest stage, though it is 

 difficult to be certain that there is still no siliceous deposit at all. 

 Scattered sparsely through the choanosome, beneath the cortex, 

 occur a number of very slender rods (figs. 1 and 8) about 0'04 

 mm. in length, which, unlike the silica which is subsequently 

 deposited around them, stain rather deeply with borax-carmine. 

 It is difficult to determine the exact shape of their extremities, 

 but in L. apicalis one end appears to be more sharply pointed 

 than the other, though this may not be a constant character. 



I have not been able to demonstrate that these protorhabds 

 arise within single mother-cells, though it is quite possible 

 that it may be so. In any case I do not think that they can 

 be looked upon merely as secretions, for in L. apicalis, as I shall 

 show presently, they retain the power of independent growth 

 long after the commencement of silica-deposition around them. 

 I am inclined rather to compare them with the plastids found 

 in many vegetable cells, in connection with which various sub- 

 stances, such as chlorophyll and starch, are deposited by the 

 protoplasm. Such plastids are living bodies, capable of growth 

 and multiplication, but they always seem to remain within their 

 parent cells. The histological conditions in the sponge are, 

 however, very different from what they are in plants, and it seems 

 almost certain that the protorhabds sometimes grow far beyond 

 the limits of any parent cell that there may be. It is the proto- 

 rhabd which, as we shall see later on, is responsible for growth 

 in length of the spicule ; and when we remember that some 

 spicules, such as those of the " glass-rope " in Hyalonema, or 

 the even more remarkable fixing spicule in Monorrhaphis, may 

 be considerably over a foot in length, it is evident that this struc- 



