Chapter XII 



EVIDENCE FAVORABLE TO CHROMATIN AS 

 "HEREDITARY SUBSTANCE" 



A. DIRECT EVIDENCE 



Evidence from the Ontogeny of Some Protozoans 



WE conform to the time-honored custom in zoology of 

 beginning the examination at the lower end of the 

 taxonomic scale. 



As a first example we take the trichocysts (tr. figurelS) 

 that occur in some protozoa of the class Ciliata. Parame- 

 cium, for instance, is well armed with these organs. Briefly 

 characterized, they are elongated, spindle-shaped bodies sit- 

 uated in the ectoplasm perpendicular to the surface of the 

 body. Each has a short bristle-like process at its outer end 

 which probably pierces the pellicula, or outermost layer of 

 the body, and is in contact with the surrounding world. 

 They are organs of offense and defense, and under proper 

 stimulation are explosively converted into long, somewhat 

 rigid threads which project from the surface of the animal. 



As the animal on which the investigation we shall make 

 use of is not Paramecmm, but a less generally known rela- 

 tive, Front onia leucas, our figures will be taken from thi? 

 latter species (figure 15). The work referred to is by C. 

 Tonniges and is so recent as to have had hardly time t< 

 receive the confirmation by other students which uncxpecl 

 results should generally get before being used in a work 

 like this. But Tonniges is an experienced and trustworthy 



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