m AXIAL FIBRE 277 



was mentioned above that while the stalk in its fully 

 expanded condition is straight, the axial fibre is not straight, 

 but forms a very open spiral, i.e., it does not lie in the centre 

 of the stalk, but at any transverse section is nearer the 

 surface at one spot than elsewhere, and this point as we 

 ascend the stalk is directed successively to all points of the 

 compass. 



Now suppose that the axial fibre undergoes a sudden 

 contraction, that is to say, a decrease in length accompanied 

 by an increase in diameter, since as we have already 

 seen there is no decrease in volume in protoplasmic 

 contraction. There will naturally follow a corresponding 

 shortening of the elastic cuticular substance which forms the 

 outer layer of the stalk. If the axial fibre were entirely 

 towards one side of the stalk, the result of the contraction 

 would be a flexure of the stalk towards that side, but, as its 

 direction is spiral, the stalk is bent successively in every 

 direction, that is, is thrown into a close, spiral coil. 



The axial fibre is therefore a portion of the protoplasm 

 which possesses the property of contractility in a special 

 degree ; in which moreover contraction takes place in a 

 definite direction— the direction of the length of the fibre — 

 so that its inevitable result is to shorten the fibre and con- 

 sequently to bring its two ends nearer together. This is the 

 essential characteristic of a muscular contraction, and the 

 axial fibre in the stalk of Vorticella is therefore to be looked 

 upon as the first instance of a clearly differentiated muscle 

 which has come under our notice amongst unicellular 

 animals. 



There are some interesting features in the reproduction of 

 Vorticella. It multiplies by binary fission, dividing through 

 the long axis of the body (P^ig. 72, e^, e^). Hence it is 

 generally said that fission is longitudinal, not transverse, as 



