chap, xi The Elements of Structure 



181 



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 (3) round about. (From Evolution of Sex; 

 after Carnoy.) 



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 slight outer 

 membrane, varying much in definiteness and sometimes 

 quite absent, through which communications with neigh- 



