Evidence from Protozoans 375 



besides chromatin enters into their production. The following 

 paragraph from Tonniges tells the story in outline. "Trichocysts 

 in the act of origination which I have designated as trichochro- 

 midia, present two substances. One is intensely colored with 

 the Dttcleat staining medium employed (ach, figure 21), so must 

 be regarded as chromatin. The other remains uncolored and con- 

 sequently is held to be achromatic substance (achr, figure 21). 

 Tin first produces the axial rod of the future trichocyst, while 

 the latter, the achromatic substance, produces the external en- 

 velope and the myoneme-like structure." 7 Four stages in the de- 

 velopment of a trichocyst are shown in figure 21 a, b, c, d. Natur- 

 ally many detailed structural changes not here noticed in both 

 the axial part and the enveloping part occur before the organ is 

 completed and ready for use. But these need not concern us 

 since they in no way affect the main point, namely that evi- 

 dence that the achromatic substance of the macronucleus is 

 the physical basis of heredit}' of organs under consideration, 

 comes from exactly the same source and is exactly as valid as 

 is the evidence that chromatin plays such a role. 



The question of whether the cytoplasm of the animal plays 

 a direct part in the development of these organs, while very 

 important were we seeking for an adequate general theory 

 of development or for complete knowledge of the factors 

 involved in this particular development, must not detain 

 us since all we are concerned with in this section is to find 

 whether any substances other than chromatin are deter- 

 miners of hereditary attributes. 



(e) The Skeleton of Radiolaria 



With these illustrations from the infusoria of sub- 

 stances other than chromatin which serve as the physical 

 basis of heredity, we must turn from the endless examples 

 that might be drawn from the same group, and pass to 

 another great sub-division of the protozoa, the Radiolaria, 

 for a few illustrations. In the chapter on the general 

 ontogeny of the protozoa we spoke particularly of Hacker's 

 studies on the development of the skeleton in the Aulo- 



