48 THE ANIMALS AND MAN 



of a viscous liquid containing many fine globules of a liquid 

 of different density. It is a sort of liquid foam. Some 

 naturalists however, believe the fine globules to be solid 

 granules while still others believe that numerous fine threads 

 of dense protoplasm lie coiled and tangled in the clearer 

 viscous protoplasm. The difficulty in determining the 

 physical structure is due to the limitations of the microscope. 

 The ultimate structure of protoplasm is ultra-microscopic. 

 What little is known of the chemistry and physics of proto- 

 plasm certainly is far from explaining its wonderful properties. 

 It should be held clearly in mind also that the full life capacity 

 of protoplasm is realized only when it is in that differ- 

 entiated and organized condition typical of the structural 

 unit or cell. The essential thing about the cell is not that 

 it has a definite shape or size or that it is truly cell- or sac- 

 like, but that it is a tiny but definitely organized mass of 

 protoplasm with various substances secreted by or held in 

 it. The protoplasm itself is differentiated into at least 

 two parts, an inner, denser, smaller part called the nucleus, 

 and an outer surrounding, usually larger, portion called 

 the cytoplasm. Such a differentiated or organized proto- 

 plasmic unit can perform all of the essential functions of 

 life and persist in this performance indefinitely unless de- 

 stroyed by extrinsic causes. The cell itself may not have 

 an indefinite existence as a unit, but it will be the progenitor 

 of an indefinitely prolonged series of cells. A single part 

 of this cell, that is, a bit of protoplasm either of the nucleus 

 or the cytoplasm, or the whole of either can perform for a 

 while most of the activities of life; but such a part always 

 lacks the capacity for reproduction, that is, for persistence 

 as living matter. 



