clxiv 
THE VOYAGE OF H.M.S. CHALLENGER. 
of temperature causes it to thaw and wrinkle up. In hot weather the process is in 
consequence not practicable except in a freezing chamber. The slices are next covered 
with pure glycerine, and a cover-glass placed over them, glycerine jelly is run round the 
edge of this, and the slide is placed in the water-oven and left till the glycerine has 
completely converted the gelatine into glycerine jelly. If this part of the process is 
delayed for some few days, the gelatine undergoes some modification by which it is 
rendered incapable of pectinising with the glycerine, and this impairs the value of the 
preparation in two ways : in the first place the refractive index of the gelatine is higher 
than that of the glycerine, and this interferes with the optical clearness of the 
preparation ; and in the next place the modified gelatine exerts in process of time a 
bleaching action on the stained tissue, and finally entirely discharges its colour. , In 
ignorance of these facts many of my earlier preparations were left too long before 
warming, and are now in consequence almost worthless. 
The value of glycerine in optically despiculising a sponge-slice has already been 
pointed out by me,^ and subsequently by Schulze, but frozen slices mounted in glycerine 
have other advantages over paraffin preparations ; for one thing the tissues sufier far less 
contraction, indeed in this respect there is no comparison possible between the two 
methods; but, still more important in the study of Sponges, comparatively thick slices can 
be cut with better results than in the case of parafiin preparations. The value of thick 
slices which have suffered only a minimum of contraction is well exemplified in the case of 
the sterrasters of the Geodiidse ; I should never have made out the scleroblast of these 
spicules in paraffin-cut slices, for with a full knowledge of what to look for I have only 
once or twice succeeded in finding them in such preparations ; while in slices obtained by 
the freezing process there need never be any difiiculty. Similarly the only traces of 
scleroblasts observed in connection with the adult desmas of the Lithistida were met 
with in slices obtained by freezing. 
Isolation of the Spicules . — In the case of the Choristida a thin fragment cut from the 
surface almost to the centre of the Sponge is placed on a glass slide and boiled in excess 
of strong nitric acid ; when all the soft parts have been thus destroyed, a triangular piece 
of blotting paper, moistened at one corner with water, is placed with the moistened corner 
touching the edge of the acid on the slide ; when most of the acid has been drawn off, 
distilled water is added to the slide from a dropping tube and drawn off by blotting 
paper in the same way, when as much as possible has been removed from the slide lying 
flat, it is raised to slope at a gentle angle and the blotting paper replaced by a dry piece, 
the angle is gradually increased till the slide stands vertically. A second washing with 
water is necessary when the fragment of Sponge operated upon is of comparatively large 
size ; when sufficiently washed with water, absolute alcohol is added and likewise drawn 
off by blotting paper. The slide is placed in the water-oven and when dry a little xylol 
1 Sollas, Ann', and Mag. Nat. Hist., ser. 5, vol. iv. p. 48, 1879. 
