WHAT IS AN ORGANISM? 1 65 



the higher forms of life are aggregates of such morphological 

 units, or cells, variously modified."* 



The primitive form of the organic individual is the simple 

 cell of microscopic size, globular in shape and with no distin- 

 guishable differences in the structure of its contained proto- 

 plasm. If higher powers of microscope could be brought to 

 bear, it is not improbable that, like the nebulae of the macro- 

 cosm, this amorphous unit of the organic microcosm might be 

 resolved into complexity; but, as we know them, cells are 

 found almost universally to possess three elements of structure : 

 (i) the protoplasmic substance of the cell, (2) a cell-wall or 

 marginal sheath, and (3) a nucleus within. 



The Three Ways by which Cell-modification is Accomplished. 

 There are three ways by which diversity of form is attained 

 by the cell : 



(1) By movement of the cell itself, exhibited in change of 

 shape of its exterior form, or of the cell-wall. This is seen 

 in the Amoeba (Fig. 5.1), which, by drawing in one part and 

 extending another, assumes various forms, temporarily, but 

 remains in the simple cell state. 



(2) The second method of attaining diversity of form is 

 by cell- division, which is the common method by which 

 growth is effected. Reproduction of a new cell is accom- 

 plished by such division of the original cell, separation of one 

 part from the other, and completion of its outlines by each 

 part until division into two distinct cells takes place. The 

 Protozoa are characterized by this mode of development, and 

 by the necessary failure to attain complexity of structure of 

 the individual, which reaches no higher stage of diversity than 

 the unicellular stage. 



(3) The third method of attaining diversity of form is by 

 cell-multiplication within the individual. 



Metazoa Characterized by Histogenesis, or the Formation of 

 Tissues. All the animals of the classes higher than Protozoa 

 are ranked together under the general name Metazoa, and are 

 distinguished from them by this differentiation of the sub- 

 stance of the body into cells. This, which is the second 



* Huxley, Ency. Brit., gth ed., vol. in. p. 682. 



