440 ON CONCRETIONS, SPICULES, ETC. [ch. 



concerned, but which owes its now somewhat comphcated form 

 to a restraint imposed by the individual cell to which it is confined, 

 and within whose bounds it is generated. The conception of a 

 spicule developed under such conditions we owe to a distinguished 

 physicist, the late Professor G. F. FitzGerald. 



Many years ago, Sollas pointed out that if a spicule begin to 

 grow in some particular way, presumably under the control or 

 constraint imposed by the organism, it continues to grow by 

 further chemical deposition in the same form or direction even 

 after it has got beyond the boundaries of the organism or its 

 cells. This phenomenon is what we see in. and this imperfect 

 explanation goes so far to account for, the continued growth in 

 straight lines of the long calcareous spines of Globigerina or 

 Hastigerina, or the similarly radiating but siliceous spicules of 

 many Radiolaria. In physical language, if our crystalUne 

 structure has once begun to be laid down in a definite orientation, 

 further additions tend to accrue in a like regular fashion and in 

 an identical direction ; and this corresponds to the phenomenon 

 of so-called "orientirte Adsorption," as described by Lehmann. 



In Globigerina or in Acanthocystis the long needles grow out 

 freely into the surrounding medium, with nothing to impede their 

 rectilinear growth and their approximately radiate distribution. 

 But let us consider some simple cases to illustrate the forms which 

 a spicule will tend to assume when, striving (as it were) to grow 

 straight, it comes under the influence of some simple and constant 

 restraint or compulsion. 



If we take any two points on some curved surface, such as 

 that of a sphere or an ellipsoid, and imagine a string stretched 

 between them, we obtain what is known in mathematics as a 

 "geodetic" curve. It is the shortest line which can be traced 

 between the two points, upon the surface itself; and the most 

 familiar of all cases, from which the name is derived, is that curve 

 upon the earth's surface which the navigator learns to follow in 

 the practice of "great-circle sailing." Where the surface is 

 spherical, the geodetic is always literally a "great circle," a circle, 

 that is to say, whose centre is the centre of the sphere. If instead 

 of a sphere we be dealing with an ellipsoid, the geodetic becomes 

 a variable figure, according to the position of our two points. 



