DEVELOPilENT OF AMPHIUEA SQUAMATA. 131 



cavities. Added to all this, the ccelom in Amphiura squa- 

 mata is represented at first by a mass of mesenchyme; and as 

 this condition of things is certainly not primitive, I do not think 

 that even if reliable results as to the development of cavities 

 originating before the coelom clears were attainable, they 

 would be of much phylogenetic importance. I hope to be able 

 to find something more instructive when I study their history 

 in a larger and more typical form. 



Methods. 



I. Fixing. — The following is a list of the preserving fluids 

 employed. Corrosive sublimate in distilled water ; corrosive 

 sublimate in sea water ; corrosive sublimate three parts, gla- 

 cial acetic one part ; chromic acid ; picric acid ; Flemming's 

 fluid j glacial acetic ; alcohol of 30 per cent. ; hot alcohol of 70 

 per cent, with a few drops of corrosive sublimate ; osmic acid 

 from about ^ to 1 per cent. ; osmic acid followed by MUUer's 

 fluid for eighteen to twenty hours. 



The result of my experiments is this : — The only liquid 

 which gives reliable results is osmic acid, though its use is 

 attended by disadvantages. It renders the animals very 

 brittle, and has little penetrating power. When used in too 

 strong solution, or for too long a time, it is followed by con- 

 siderable shrinkage. I have found, however, that when it was 

 followed by Miiller's fluid this shrinkage was entirely pre- 

 vented, and the brittleness, though still considerable, was very 

 much less. Alcohol I rejected on account of the imperfect 

 preservation it gave. Corrosive sublimate would give very fair 

 results if it were possible to render its solution quite neutral, 

 but this is very difficult. In no case, however, could one 

 obtain the beautiful diS'erentiation of tissues as to their stain- 

 ing properties which is yielded by osmic acid. 



All liquids which decalcified as well as fixed were of no use, 

 on account of the cavities which the evolution of gas produced 

 in the still soft tissues. The method I finally adopted was as 

 follows : — The animals were killed in a solution of osmic acid 

 of about ^ per cent, allowed to act for ten minutes or more, of 



