THE BIOLOGY OF THE CELL SURFACE 



which attain great length. Even the simplest unipolar nerve- 

 cell, that is, a cell with one prolongation, is a cell in which 

 the ectoplasm is drawn out into an extensive filament. 

 The more richly branched cells and those with the longest 

 prolongations show no cytoplasmic inclusions in the 

 branches, i.e., the fibres; instead, the cytoplasmic inclusions 

 are located in what is called the nerve cell body. Struc- 

 turally, therefore, the nerve cell is not only rich in surface, 

 but also in a special kind of ectoplasm. It is more than an 

 accident that this structure appears in highly conducting 

 tissues. The minute attenuated threads of nerve fibres 

 are admirably adapted for transmitting impulses. 



I have already mentioned the work of Harrison who 

 demonstrated that the growing ner^'e fibre shows definite 

 surface changes at the growing tip.^ This tip eventually 

 becomes the means by which one nerve cell establishes con- 

 tact with another, the junction being known as the synaptic 

 membrane. The fibre from one nerve cell does not pierce 

 the cell body of another, but simply comes into delicate 

 contact with it; and so the impulse passes from one nerve 

 fibre to another. Conduction thus is carried on primarily 

 by the fibre. 



Conduction in nerve cells, in other words, being a peri- 

 pheral phenomenon, is not unlike that in an egg cell, since 

 the interior of the egg cell is not involved in the conduction 

 of the fertilization-stimulus. In free living unicellular ani- 

 mals one can not always clearly identify conduction as a 

 surface phenomenon because often conduction is followed 

 so closely by contraction that it is difficult to separate the 

 two processes. Nevertheless, there are forms in which it 

 can be shown that the conduction is purely a surface 



phenomenon. 



It is to be borne in mind that conduction by nerve cells 



in higher organisms is not restricted to one cell. In the 



^ Harrjso7i, I.e. 



120 



