570 E. A. MINCHIN. 



displaced from their position ; even in the most successful 

 preparations the filaments have a more or less undulating 

 appearance (Fig. C), and show signs of displacement. The 

 processes which a piece of tissue goes through in its passage 

 from alcohol to paraffin are such as to set up a great many 

 diffusion currents in the tissues, and in consequence^ if the 

 spicules are decalcified, the axial filaments are in nearly every 

 case displaced and stuck against the spicule sheaths, and 

 undistinguishable from them after staining. 



The simplest method of demonstrating the axial filaments is 

 the following. Take any spirit specimen of an Ascon, and put 

 a small piece — a tube or part of one — first in water and then 

 in the staining mixture for about half an hour. Then wash 

 thoroughly in water, and pass through alcohol to clove oil. 

 In the latter medium the tube can be split open with needles, 

 and the collar-cells removed with a paint-brush, which generally 

 brings away the pore-cells as well. Then the piece of the wall 

 is laid out flat on a slide, and mounted in Canada balsam. I 

 have never failed to demonstrate the axial filaments in the 

 clearest manner in such preparations. 



I also made the attempt to unmount and stain some of my 

 preparations mounted in glycerine, but without great success. 

 The stain in these cases acted feebly, and the delicate filaments 

 could only be made out with great difficulty. On the other 

 hand, the sheaths were well shown, and it could be seen that 

 they appear very early in the development of the spicule when 

 it is still quite enveloped in its cells. At first the sheath forms 

 an excessively thin layer, which gets thicker as the development 

 proceeds. In such a stage as that shown in PI. 38, fig. 7, the 

 sheath is perfectly distinct when stained, a fact that leaves 

 no doubt as to its being, like the spicule, an intra-cellular 

 secretion. 



The axial filament runs to the extreme tip of the spicule ray, 

 and joins the spicule sheath (woodcut. Fig. C), showing that 

 the spicules must be perforated at the tips of the rays. The 

 filaments are more distinct towards the tips of the rays than at 

 the bases, and the behaviour of the filaments at the centre of 



