STIU'CTl'RAL CHARACTERS OF ANIMAL ORGANISMS. 35 



they were the outcome of changes clue to the activity of the pro- 

 toplasm, and that this latter was the only real essential and vital 

 part of the cell. 



Subsequently, from the fact that some vegetable cells in the 

 youngest and most active stage of their growth have no limiting 

 wall, and that most animal cells have none during any part of 

 their life, it was proposed to define a cell as a mass of protoplasm 

 containing a nucleus. But. further research showed that the nu- 

 cleus was not always present. In many cryptogamic plants no 

 nucleus can be found, and in some animal cells, which must be 

 regarded as independent individuals (Protamceba), there is no 

 nucleus at any part of its lifetime. This would lead us to suppose 

 that a mass of protoplasm capable of manifesting all the phenomena 

 of life would be a sufficient definition. Though this is probably 

 correct in a few cases, the vast majority of cells do contain nuclei. 

 As it is difficult to divest our minds of the connection between 

 the two, it has been proposed to give the name cytode to the non- 

 nucleated forms, which certainly are very exceptional, reserving 

 the term cell for the common nucleated unit. Each part of the 

 cell may now be considered in the order of its importance, viz., 

 protoplasm, nucleus, cell wall and cell contents. 



I. Protoplasm is a colorless, pale, milky, semi-translucent 

 substance, more or less altered in appearance by various foreign 

 matters which lie in it. These latter also give it a granular ap- 

 pearance, and when dead it commonly exhibits a linear marking 

 or fine network. During life its consistence is nearly fluid, vary- 

 ing, with the circumstances in which it is placed, from that of a 

 gum solution to a soft jelly. When living unmolested in its nor- 

 mal medium it seems to flow into various shapes, but this is a 

 living action which does not prove it to be diffluent, for any at- 

 tempt to investigate it by experiment causes a change in its con- 

 sistence approaching to rigidity. 



As the full comprehension of the function of this substance lies 

 at the root of the greater part of Physiology, the reader is referred 

 for a detailed account of its properties to Chapter III on Vital 

 Phenomena, where it will be discussed at greater length. 



LI. The Nucleus. Most independent masses of protoplasm, 



