IX] OF INTRACELLULAR SPICULES 675 



The conception of a spicule developed under such conditions came 

 from that very great mathematical physicist, 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 Hnes of the long calcareous 

 spines of Globigerina or Hastigerina, or the similarly radiating but 

 siliceous spicules of many Radiolaria. In physical language, if our 

 crystalline 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: corresponding to the phenomenon 

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



In Globigerina or in Acanihocystis the long needles grow out 

 freely into the surrounding medium, with nothing to impede their 

 rectihnear growth and approximately radiate symmetry. 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 some simple and constant restraint or compulsion. 



If we take any two points on a smooth curved surface, such 

 as that of a sphere or spheroid, and imagine a string stretched 

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

 ''geodesic" curve. It is the shortest Une which can be traced 

 between the two points upon the surface itself, and it has always 

 the same direction upon the surface to which it is confined; the 

 most famihar of all cases, from which the name is derived, is that 

 curve, or "rhumb-line," upon the earth's surface which the navi- 

 gator learns to follow in the practice of "great-circle sailing," never 

 altermg his direction nor departing from his nearest road. Where 

 the surface is spherical, the geodesic is Hterally 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 a spheroid, whether 

 prolate or oblate (that is to say a figure of revolution in which an 

 ellipse rotates about its long or its short axis), then the system of 

 geodesies becomes more complicated. For in it the elliptic meridians 

 are all geodesies, and so is the circle of the equator; though the 



