574 E. A. MINCHIN. 



regard to the time at which the rays become united, as mea- 

 sured by the length of the rays, would require correction. 



The results of my investigations, in which I have to acknow- 

 ledge much valuable assistance from Professor Miers, will be 

 found below in tabulatory form ; but I will first briefly indicate 

 the conclusions to be drawn from them. The chief point of 

 interest which I have discovered is that the spicule rays 

 are not crystalline when first laid down; so long as 

 the rays are quite separate they will not light up when rotated 

 between crossed nicols.^ After union of the rays they gradually 

 become crystalline, the change appearing to start from the 

 centre, and in fact from the secondary deposit of calcite by 

 which the rays are united together. I have seen many appear- 

 ances which indicate that the rays are united by a disc or 

 globule of calcite deposited at the centre over the apposed 

 extremities of the rays. In some preparations of cerebrum, 

 viewed from the dermal surface, one sees at a lower focus 

 the interspaces between the rays ; at a higher focus a circular 

 piece of calcite comes into view, sometimes so distinct that it 

 looks like the rudiment of another ray. This central portion 

 seems to be in many cases the part that first lights up between 

 crossed prisms. On the other hand, spicules may be found 

 which have the rays completely joined, but which fail to show 

 any crystalline properties ; others, again, light up so feebly 

 between the prisms that it is very difficult to detect the feeble 

 illumination, except by observing the slight degree to which it 

 waxes and wanes as the stage is rotated. On the other hand, 

 amongst and between spicules which remain dark there 

 occur others scarcely differing in size, or even smaller, which 

 light up with the greatest brilliancy, thus supplying a valuable 

 contrast. 



The next result which merits special notice is the fact that 

 the period in the development at which the rays assume a 

 crystalline character varies greatly in diflPerent species, in 



1 As this result would also be produced if the optic axis of the crystal 

 coincided with that of the microscope, I have, where it was possible, tilted 

 t|)e preparation, and found that the spicules still remained dark. 



