200 



THE STUDY OF ANIMAL LIFE CHAP. 



and sometimes quite absent, through which communica- 

 tions with neighbouring cells are often established ; and 

 (4) of non-living cell contents, products of the vital 

 activity such as pigment and fat. Most animal cells 

 also contain a centrosome which plays an important part 

 during cell-division. The general cell-substance or cyto- 

 plasm is sometimes, as it were, colonised, by minute 

 structures of nuclear origin called chromidia, made of 



FIG. 53. ANIMAL CELL, SHOWING THE COILED CHROMATIN THREADS OF 

 THE NUCLEUS (a), AND THE PROTOPLASMIC NETWORK (&) ROUND ABOUT. 



(From Evolution of Sex ; after Carnoy.) 



material comparable to the chromatin of the chromo- 

 somes of the nucleus. Moreover, the cytoplasm often 

 shows specialised bodies called mitochondria which are 

 connected with particular cellular functions, such as 

 secretion. These things are mentioned so as to give 

 at least a hint of the extraordinary complexity of struc- 

 ture that there often is in a cell much smaller than a 

 pin-prick. 



Two very interesting general facts in regard to the 



