198 



THE STUDY OF ANIMAL LIFE CHAP. 



of living matter or cells. 

 From 1838-39, when 

 Schwann and Schleiden 

 stated in their " cell doc- 

 trine " that all organisms - 

 plants and animals alike 

 -were built up of cells, 

 cellular biology may be said 

 to date. It was soon shown 

 as a corollary that every or- 

 ganism which is reproduced 

 in the ordinary fashion 

 arises from a single egg-cell 

 or ovum which has been 



FIG. 51. A SINGLE CELL OF 

 CILIATED EPITHELIUM TO IL- 

 LUSTRATE A LITTLE OF THE 

 COMPLEXITY THAT THERE MAY 

 BE IN CELLS. 



(After Schneider.) 



At the roots of the cilia (C) there 

 are basal corpuscles (B), and from 

 these there are fine root-threads ex- 

 tending far into the cell. The nu- 

 cleus (N) shows a nucleolus and 

 small chrqmatin granules. The 

 cytoplasm is shaded with dots. 



fertilised by union with a 

 male-cell or spermatozoon. 



Now, the cells of the ani- 

 mal bodv are necessarilv 



*/ / 



varied, for the existence of 

 a body involves division of 

 labour among the units. 

 Some, such as the ciliated 

 cells lining the windpipe, 

 are very active, like the 

 Infusorian Protozoa ; others, 

 for instance fat-cells and 

 gristle-cells, are very pas- 

 sive, something like the 

 Gregarines ; others, such as 



