60 PRINCIPLES OF ANIMAL BIOLOGY 



determinate in size, the size of any particular kind of cell in any organism 

 being fairly uniform. When the cell has reached the limit of its size it 

 may divide. The half-sized cells then grow to normal size. Thus the 

 growth of the cell is due to the increase in the quantity of protoplasm. 

 Increase in size of many-celled organisms is usually due to maltiplication 

 of cells and to the growth of the half-sized cells to normal. The cells of a 

 large frog are no larger than the cells of a small frog of the same species, 

 but there are more cells in the large frog. Increase in the size of the cell 

 may not be wholly due to increase in the quantity of protoplasm. Fat- 

 cells increase in size because of the deposition of globules of fat, a process 

 which may be continued until there is much more fat than protoplasm. 

 In plant cells and certain animal cells volume may be increased by the 

 imbibition of water which may be stored in vacuoles. In such extreme 

 cases as those mentioned the quantity of protoplasm may be actually 

 decreased although the cell may be larger. In every case where growth 

 occurs it is due to the activity of the protoplasm. This is true in multi- 

 cellular as well as unicellular organisms. 



Reproduction. — Reproduction is likewise characteristic of living 

 beings. In unicellular organisms, and only in these, reproduction is 

 equivalent to cell division, for obviously a cell cannot reproduce its 

 kind without a cell division of some sort. In higher organisms repro- 

 duction usual]}'- involves the formation of special cells, the germ cells, 

 which by their division, with rearrangement of the resulting cells, give 

 rise to new organisms. Here reproduction involves cell division too. 

 An account of cell division is given in Chapter IV and a detailed 

 account of reproduction in Chapter VIII. 



Protoplasmic Movement. — One of the attributes usually ascribed to 

 living organisms, distinguishing them from non-living matter, is the 

 power of independent motion. This power resides in the protoplasm. 

 Protoplasmic movement may result in locomotion, that is, change of 

 position of the organism in space, or it may be confined to a change of 

 position of particles of protoplasm within the cell itself with no resulting 

 locomotion. Most animals at some stage in their existence, many plnnts 

 of the lower orders and the swarm spores of other low plants are motile. 

 Higher plants are not capable of locomotion, but within their cells the 

 protoplasm may undergo movement such as streaming or flowing. Naked 

 plant cells and some forms of unicellular animals frequently progress by a 

 type of protoplasmic movement which is so characteristic of the protozoon 

 Amcjeba that it has come to be called anurhoid movement. Other cells 

 make use of cilia which are minute slender protoplasmic processes capable 

 of rapid vibration. This movement of cilia is ciliary movement. Loco- 

 motion i n most higher animals is due to the movement of appendages caused 

 by contraction of special contractile cells, tlie muscle cells. Such move- 



