MATERIALS FOR A MONOGRAPH OP THR ASOONS. 569 



iiective-tissue layer distinct in function from the contractile, 

 undifferentiated dermal epithelium. In Leucosolenia also 

 the actinoblasts of the triradiate systems form a deeper layer, 

 but the dermal epithelium secretes primary monaxons — at 

 least in the young form — and is non-contractile. 



6. The forms of the spicules are the result of adaptation to 

 the requirements of the sponge as a whole, produced by the 

 action of natural selection upon variation in every direction. 



ADDENDA. 



A. On the Presence of an Axial Organic Filament 



in the Spicule Rays. 



(Note to p. 543.) 



Since the above paper went to press I have succeeded in 

 demonstrating very clearly the existence of an axial organic 

 thread in the spicule rays by means of the following staining 

 mixture, recommended to me by my friend Dr. Ritchie, Lec- 

 turer on Pathology in the University of Oxford : 



Nigrosin, 1 per cent, in ILO . . .1 volume. 

 Picric acid, sat. sol. in H^O . . .9 volumes. 



The action of this mixture upon the sponge tissue is that, in 

 the first place, the calcite of the spicules is rapidly dissolved by 

 the picric acid, which also colours the cytoplasm of the cells 

 yellowish, while the nigrosin stains the ground substance a 

 faint blue and the spicule sheaths deep blue, and also colours 

 a very delicate filament in the axis of each spicule ray. 



For showing the filaments, sections of material which has 

 been decalcified before embedding are almost useless; on the 

 other hand, the filaments are seen very well in sections not 

 previously decalcified after treatment with the nigrosin mixture, 

 especially in fairly thick sections, in which it is possible to find 

 large pieces or whole rays of the spicules. I attribute this 

 diflFerence to the fact that after decalcification the filaments are 

 quite unsupported within the spicule sheaths, and are readily 



