CHAP. XI The Elements of Structure 



i8i 



among the units. Some, such as the lashed cells lining 

 the windpipe, are very active, like the Infusorian Protozoa ; 

 others, for instance the fat -cells and gristle-cells of connective 

 tissue, are very passive, something like the Gregarines; 

 others, such as the white blood corpuscles or leucocytes, 

 are between these extremes, and resemble the amoeboid 

 Protozoa. 



But it is true of most of them that they consist (i) of a 



Fig. 34. — Animal cell, showing the coiled chromatin threads 01 the nucleus (a), 

 and the protoplasmic network (i) round about. (From Evolution of Sex ; 

 after Carney.) 



complex, and- in part living cell -substance, in which keen 

 eyes looking through good microscopes detect an intricate 

 network, or sometimes the appearance of a fine foam ; (2) 

 of a central kernel or nucleus, which plays an important 

 but hardly definable part in the life of the cell, especially 

 during the process of cell-division ; (3) of a shght outer 

 membrane, varying much in definiteness and sometimes 

 quite absent, through which communications with neigh- 



