VITAL PROPERTIES OF CELLS 17 



(c) Molecular motility is a dancing or oscillatory movement of the 

 granules in living protoplasm. Such granules may be non-living matter, 

 pigment, etc. This type of motion is also called brownian movement. 

 It is probably purely a physical phenomenon. It may be simulated by 

 mixing finely divided carmin with glycerin. 



(d) Circulatory or streaming movement is present in various de- 

 grees in probably all living protoplasm. It is only when it is rapid 

 that it becomes easily discernible. It is readily demonstrable in certain 

 cells, e.g., chara and nitella; also less readily in certain protozoa (Para- 

 mecium). It must most probably be interpreted as a form of respira- 

 tion. It is characterized by a flowing 



or streaming of the protoplasmic gran- 

 ules in a definite direction. 



(e) The reason for listing muscu- 

 lar as a separate type of motility is 

 mainly its predominance in animals 

 and the fact that it does not apparently 

 fully conform to any of the above types. 



It is characterized by a reversible FlG . 2 3,-CiLiATE AND FLAGELLATE 



process of contraction of specially dif- CELLS. 



ferentiated muscle fibrils. It perhaps A, ciliated cells isolated from the 



most closely resembles streaming motil- trachea of a cat; B, human sperma- 



, T, -i -, i , / tozoa 1. in surface view; 2, in 



ity. It leads to least confusion, in view profile ' Examined fresh in ' no ; mal 



of our present lack of definite knowl- saline solution, x 550. 

 edge regarding the physical and chem- 

 ical phenomena underlying muscular motion, to speak of it as a distinct 

 type. It will be further discussed under Muscle. 



(4) Reproduction. The essence of reproduction is cell multiplica- 

 tion. A living cell has the power of producing other cells like itself. 

 Viewed philosophically, cells may conceivably arise in two different ways : 

 (1) from non-living material, spontaneous generation (abiogenesis) ; (2) 

 from preexisting cells by division. Science has quite generally accepted the 

 aphorism 'omnis cellula e cellula' (Virchow) as an expression of the 

 whole truth. However, full acceptance of the doctrine of evolution logi- 

 cally compels belief in spontaneous generation : this not in any such 

 crude form -as that frogs may arise from the mud of rivers, or insects 

 from dew or dung, but that given the conditions (conceivably possible 

 somewhere in the universe to-day) prevalent when life first appeared as 

 the original mass of living protoplasm, the 'cytode' or 'cytoblastema/ the 

 inorganic may continually be passing into the primarily organic, e.g., 



