APPENDIX B 313 



cell must be greater than that for the whole egg. What the 

 mathematical values of these probabilities are Boveri does not 

 know, though he makes an attempt to reckon them not 

 theoretically, but by means of a mechanical apparatus; the 

 attempt is not quite successful. The fact, however, remains that 

 eight per cent, of the triasters produce normal Plutei, only -06 per 

 cent, of the tetrasters. This does not depend on the cells receiving 

 too much or too little chromatin (see p. 265), nor again on the 

 fact that the ratio between size of nucleus and size of cytoplasm 

 (see pp. 268, 269) can only be satisfied by certain definite 

 numbers of chromosomes, and the only explanation remaining is 

 that for normal development of each and every part the nucleus of 

 each cell must contain a complete set of the specific chrosomomes ; 

 from which it follows that the chromosomes are qualitatively 

 unlike. 



A word may be said about the double-spindled eggs (Type 

 A. i). The larvae from these sometimes show abnormal regions, 

 and this is attributed to one or more of the binucleate cells 

 having divided with a tetraster and irregular distribution of 

 chromosomes. Of all such eggs 50 % gave rise to normal Plutei. 



The degenerative changes undergone by the nuclei of these 

 larvae are of several' types, to be associated again with differences 

 in the combinations of chromosomes. 



(ii) Boveri's experimental proof of the qualitative difference 

 of the chromosomes does not of course of itself involve a belief 

 in the individuality of these bodies, for if the chromatin is 

 concerned in inheritance, it is necessary to suppose that the 

 number of qualitatively distinct bodies is far greater than the 

 number of chromosomes, and these bodies may be differently 

 grouped during each successive resting stage. 



The hypothesis of the individuality of the chromosomes, i.e. of 

 a constancy in the manner of grouping of these particles, rests 

 in the first instance on such facts as those observed by Sutton in 

 Brachystola, where in the spermatogonia the chromosomes are of 

 different sizes, which may however be arranged in pairs, together 

 with an odd one or accessory chromosome. In the resting stage 

 the accessory chromosome remains apart in a separate vesicle, 

 while the large chromosomes lie in separate pockets of the 

 nuclear membrane, the small ones, each as a separate reticulum, 

 in the main body of the nucleus. In the spermatocyte a number 

 of bivalent spiremes appear, which show the same differences of 

 sizes as the pairs of chromosomes previously, and the accessory 

 chromosome. 



The accessory chromosome passes into two only of the four 

 spermatids and is supposed to be a sex-determinant. 



