: 
STUDIES IN SPICULE FORMATION. 251 
end. This truth also renders it more easy to understand the 
causes of variation in size of calcareous spicules, also to be 
referred to presently. 
_ As regards the secondary migration of the basal cells of 
triradiates, and the distal cells of monaxons when their power 
of excretion is exhausted, there is nothing more remarkable 
in the phenomenon than in the re-assumption of locomotive 
powers by an amoeba or infusorian after feeding or being 
otherwise engaged, and the spicule ray evidently serves as a 
guiding path: the stimulus to movement in the latter case is 
possibly the same as that in the former. It may also be 
pointed out as a possibly significant fact that the gastral 
rays, which are alone directly immersed in the surrounding 
medium, are alone among spicule rays wholly enveloped by 
the cell-substance; on calling to mind that the external 
portions of protruding monaxons never possess a cell on their 
surface, it seems possible that a connection exists between 
these two phenomena. 
Part II, The Spicules of Calcareous Sponges in general ; 
Theoretical Considerations. 
ConDITIONS AND Features oF LimME SECRETION IN CALCAREA. 
Although owing to lack of information with regard to the 
chemical and physical aspects of lime secretion, it is as yet 
difficult to definitely account for many minor features of spicule 
formation, it is yet possible to indicate the main features of 
the process and such it is now desirable to do if we wish to 
attain to a true conception of the evolution of the calcareous 
skeleton of sponges. 
The first general and obvious condition essential to the 
deposition of lime—the first law of spicule formation—is the 
proximity of the cell-substance to the area over 
which calcareous matter is being secreted.  Iilus- 
