THE CELL 



25 



a specific activity is carried on. The distinction implies that a 

 cell part, when regarded as a place where, may change its position 

 or disappear entirely, while an anatomical organ must persist. 



Ludford points out the importance of cell inclusions in diag- 

 nosis. The so-called virus bodies are such inclusions which, to 

 some extent at least, differ from the bodies usually occurring 

 in cells. They occur principally in association with disease, 

 mostly those caused by viruses such as vaccmia (cowpox), rabies, 

 measles, scarlet fever, diseases of the fowl, etc. In rabies and 

 fowl pox, the cell inclusions or virus bodies occur with such 



Fig. 17. — a, An Ascidian egg just before fusion of the male (cfn) and female 

 (9n) nuclei, showing the two asters (star-like bodies), in the center of each of 

 which is a centrosome; b, cleavage of an animal egg (Ciona) showing chromosomes 

 attached to spindle fibers and migrating toward the poles (the centers of the 

 two asters). {From E. G. Conklin.) 



constancy and are so characteristic that they are of diagnostic 

 value. The best known among these inclusions are the Negri 

 bodies. 



Fixation Artifacts. — Most of the cell structures which have 

 been and are about to be described here were first discovered in 

 material prepared, or, as we say, fixed (killed with reagents such 

 as alcohol or formaldehyde), and stained. Such cells are dead, 

 and the protoplasm coagulated. There naturally arises the possi- 

 bility that the structures present are artifacts (i.e., are artificially 

 produced). But as most of these structures have since been seen 

 in the living state, doubt as to their reality no longer exists. A 

 number, however, are still on the doubtful list in the minds of 

 some workers ; among these are the centrosomes, Golgi apparatus, 

 and spindle fibers. The existence of spindle fibers is questioned 



