FOEMS OF SPICULES. 57 



cellSj i. e. cells not derived from the original mother cell oi' 

 cells concerned in their production (spicules of the pluteus 

 larva^ lithistid desmas, etc.) are termed secondary spicules. 



Are the Forms of Spicules Inherited ? 



In considering the possible factors giving rise to the various 

 forms which spicules assume, it will facilitate matters, more 

 especially as showing in what direction the factors are to be 

 found, if it can possibly be decided first of all whether or no 

 the forms of spicules are inheritable. But previously to dis- 

 cussing this particular subject we must have clear ideas as to 

 what exactly we mean when we decide that any particular 

 skeletal or other structure is inherited. Heredity must after 

 all be regarded as a complex of physical causes identical in 

 nature with those which produce purely inorganic phenomena, 

 and, mentally, at any rate, we must carefully distinguish these 

 physical causes constituting heredity from the physical 

 (ontogenetic) causes giving rise to non-inheritable organic 

 structures, though this is admittedly usually very difficult to 

 do in fact. No one will deny that the form of a bone, tooth, 

 nail, valve of a diatom or other similar non-living part of an 

 organism is inherited, but in asserting this we evidently must 

 mean that the form of the protoplasmic mould which produces 

 the bone, tooth, nail, or valve is the part inheritable, since 

 the non-living matter composing these structures being unable 

 to reproduce itself obviously cannot inherit properties. Evi- 

 dently then to assert that the form of a siliceous or calcareous 

 spicule is inherited implies that the disposition of the sclero- 

 blasts associated with any given type of spicule is that which 

 is inherited, and that the spicule itself, like the bone or 

 diatom-valve, is simply deposited in a mould already formed 

 for it by the scleroblasts. That is, the disposition of the 

 scleroplasm on this hypothesis determines the form of the 

 spicule and not vice versa.^ 



^ "I must rather maintain that the form of the sponge spicules is deter- 

 mined by the orgaQic matrix in and from which they originate, and that the 



