The Cilium. 203 



reversible cilia both fibrils must possess the power of contracting, 

 each alternately or under different stimuli, acting as elastic support 

 for the other. 



As I have watched and studied cilia and other contractile fibrillae 

 for the past four years, comparing at every step the living with the 

 fixed structure, I have observed many things that cannot be detailed 

 in this paper, but I have seen nothing which contradicts the fibrillar 

 theory of contractile protoplasm. 'No one who has watched the basal 

 cilia of Vorticella protrude and be absorbed, often within a few sec- 

 onds, can deny great plasticity to the contractile fibril ; but, while it 

 contracts and lashes the water or pulls hard enough to pinch a para- 

 moecium in two, formed it must be, and to think of it as a strand of 

 fluid baffles imagination. No one who has watched an amoeba divide 

 can deny to the fibrils the power of auto-section or amputation, proba- 

 bly the power to liquefy at certain points ; nor can one who has 

 watched a Vorticella attach itself and grow its stalk question that the 

 contractile fibrils have the power to cement themselves to foreign sub- 

 stances, and this, it would seem, must carry the conclusion that they 

 fuse with one another. On the other hand, those who hold that con- 

 tractile protoplasm is a complex fluid and that all fibrillar structure 

 demonstrated in it is the result of fixing reagents, must explain why 

 a reagent which preserves fibrillae outside the cell might not preserve 

 them within the cell, and also why a reagent that destroys the 

 fibrillae outside might not be expected to change those inside the cell. 

 They must also bring forward a satisfactory explanation for the 

 fibrillae which can be so clearly demonstrated in cilia and flagella of 

 the living cell. 



Conclusions. 



1. Osmic acid is a satisfactory fixing reagent for the contractile 

 structures investigated. 



2. To interpret structures inside a cell after fixing with any re- 

 agent we must take into account alterations produced by it in fibrillar 

 structures outside the cell. 



3. Absence of fibrillar structure may mean that the fixing reagent 

 used has destroyed the fibrillae. 



4. The cilia of Stylonychia are comj)osed of spirally coiled fibrils; 



