16 COMPARATIVE AI^ATOMY. 



§ 14. 



In the indifferent condition, that is as long as changes in a 

 definite direction do not arise, leading to the formation of definite 

 new structures, the cells of all animal organisms appear to have 

 essentially the same character. In them we distinguish according 

 to what has been remarked above : first, the protoplasm, which 

 forms the principal mass of the body of the cell ; and secondly, the 

 cell-nucleus, surrounded by, and different from, the protoplasm, than 

 which it is usually more dense. The share which this nucleus takes 

 in many varied phgenomena of the life of the cell, compels us to 

 regard it as by no means a subordinate part of the cell-body. In 

 addition to these parts of the cell, some persons recognise (formerly 

 everyone did so) a membrane which differs from the protoplasm or 

 contents of the cell, and envelopes it : from its presence arose the 

 notion of the " vesicular form " of the cell and its name. 



Although it cannot be denied that iu many ceUs thei-e are 

 envelopes diffei-entiated from the protoplasm, yet this condition is 

 never found in the earliest life of the cell, but is always the re- 

 sult of an advanced change, and of the passage of the cell into a 

 differentiated form. 



Automatic movements of the protoplasm of the cell are 

 such common manifestations of their life, that they are always 

 definitely apparent as a property of all cells which are not highly 

 differentiated, that is of cells in which the protoplasm is not meta- 

 morphosed. In free cells, and such as are not enveloped by firm 

 membranes, this phaenomenon of movement produces locomotion. 

 Even in cells that are not free movement may be observed, consisting 

 partly in a change in the form of the surface, partly in a change 

 in the position of the granules in the protoplasm. That there are 

 also properties resident in protoplasm which we may attribute to 

 sensibility of a very low grade results from many experiments 

 and observations. 



We may further observe nutrition in the cell. At times, indeed, 

 we can see the protoplasm ingesting food; in all cases the growth 

 of the cell is an express indication of its nutrition. This ph^eno- 

 menon of growth, which may be seen in any indifferent cell, is 

 expressed by the increase in size of the protoplasmic body, owing to 

 the assimilation of matter from without. The growth of the cell 

 may be quite regulai-, increase in size obtaining in all directions, as 

 is the case with all cells in their earliest stages ; as long as this 

 lasts the cell completely retains its spherical form, unless its move- 

 ments or external influences modify it. Or growth may be unequal, 

 and the cell become elongated by increase in size along one axis ; 

 or, again, it may become stellate by growing along several axes. 

 Irregular increase of this kind is ordinarily accompanied by differen- 

 tiation of the cell-substance, and is therefore the commencement of 

 the conversion of the cells into tissue. 



