THE president's ADDRESS. 243 



menced. This forward growth is very clearly shown in fig. 16, 

 representing a stage in which the terminal portion of the 

 protorhabd projects quite freely beyond the apical knob. It is 

 noteworthy that the uncovered portion of the protorhabd was 

 deeply stained with borax -carmine, which had not been^ able 

 to penetrate through the silica coating lower down. The de- 

 position of silica follows the growth of the protorhabd, the final 

 result being the long, tapering end-piece of the adult spicule. 



The later stages in the development of the whorls are not 

 easy to follow, owing to the difficulty of obtaining anything but 

 a side view. In both species the median and the subsidiary 

 whorl appear to consist normally of three segments, each 

 derived from one of the three knobs which constitute the whorl 

 in the young spicule. In the immature stages, however, there 

 appears to be a good deal of variation in the number of these 

 knobs, as w^ill be seen by references to the figures. This apparent 

 variation may be in part illusory and due to the concealment 

 of one of the knobs behind the shaft. It does not seem to be 

 entirely so, however, and it appears probable either that one or 

 more segments of a whorl may be suppressed or that they do 

 not necessarily all develop simultaneously. Such differences 

 must depend upon differences either in the number or in the 

 activity of the formative cells. How the knobs are transformed 

 into the flattened segments of the adult whorl, with their beauti- 

 fully crenate margins (fig. 24), is entirely unknown. 



The apical whorl appears first as an equatorial ridge on the 

 apical knob (figs. 6, 16, 18). This grows out into a disk, also 

 with crenate margins (fig. 23) but not divided into segments. 

 The mechanism of this development is again quite unknown, 

 as also is that of the development of the various spines with 

 which the apex and the manubrium are ornamented, but I am 

 inclined to think that formative cells are again concerned as 

 architects in these later stages, and it seems not impossible that, 

 as already suggested, many formative cells may arise by divi- 

 sion of the original ones. 



The foregoing account of my observations on Latrunculia, 

 though far from being complete, seems to justify the statement 



