B oven's Theory 231 



the smallest visible granules succeeds only rarely. In the 

 nuclei of an American salamander, Batrachoseps, the 

 members of the nuclear threads are most distinct ; at least 

 Gustav Eisen succeeded in making an approximate count 

 of the smallest visible granules. In every pronucleus they 

 form 12 chief parts, the so-called chromosomes. Every 

 chromosome showed as a rule a subdivision into six sec- 

 tions or chromomeres, and every chromomere, in turn, 

 appears again to be built up of six smallest granules, the 

 chromioles. All in all there are here then about 400 dis- 

 tinguishable particles in the individual pronucleus. The 

 number of hereditary characters must certainly be much 

 higher than 400 for such an organism; it would more 

 likely have to be estimated at ten times that value. We 

 must therefore be satisfied, for the present, with the ob- 

 servation of groups of units in the nuclei. 3 



In the end there will surely be found a way of seeing 

 the individual units also. But the resolving power of our 

 microscope will finally reach its limit, and we shall prob- 

 ably never be able to see much smaller granulations than 

 the smallest elements that are visible now. So far, even 

 the causes of many contagious diseases, in plants as well 

 as in animals, are still quite invisible. But the calculations 

 which Errera has lately made on the limits of the smallness 

 of organisms still allow us full play. In Micrococcus he 

 finds a structure composed of about 30,000 protein mole-, 

 cules, but many nuclei are much larger. It cannot yet be 

 estimated of how many molecules a whole nuclear thread 

 is composed, but it may be assumed with certainty that not 

 every one of its granules has such a complicated structure 

 that it could hold the factors for all peculiarities of the 



3 Cf. Translator's Preface, p. viii. 



