xm] DIFFERENCE OF FUNCTION 203 



of - - = , in each nucleus. In a normally fertilised egg 



4 2 

 there are of course 2n in each nucleus of the segmenting egg, 



but the phenomena of artificial parthenogenesis show that 

 half this number (n) is sufficient to allow of normal develop- 

 ment. If, therefore, the chromosomes were functionally all 

 alike, every cell that receives as many as n chromosomes 

 should be capable of normal development. If, however, the 

 chromosomes differ from one another in function, it is 

 unlikely that a cdmplete set will be included in each of the 

 four nuclei resulting from the quadripolar spindle of a di- 

 spermic egg, if the three sets of chromosomes are distributed 

 on the four spindles at random. In the accompanying dia- 

 gram, reproduced from BOVERI, the number n is represented 

 for convenience as four (in Echinoids it is really about eigh- 

 teen), and the chromosomes from the three germ-nuctei are 

 represented by a l ,b 1 ,c 1 ,d l ,a 2 ,b 2 ,c 2 ,d 2 ,a 3 ,b 3 ,c 3 ,d 3 . If, now, 

 the chromosomes are distributed on the spindles in some such 

 way as in the diagram, only one of the four daughter nuclei 

 (the left lower) will receive all four kinds of chromosomes 

 (# , , <:, d) ; the left upper has no b, the right upper no ^, and 

 the right lower neither a nor c . This imaginary example is 

 probably an extreme one, but when the full (reduced) com- 

 plement is eighteen, it; is quite likely that one or more 

 of the nuclei will lack some members of the complete series, 

 although each nucleus contains not less than n chromosomes 

 in all. If, then, simply n chromosomes were required for 

 normal development, each of the four cells should be able 

 to continue its development in the typical way, but if the 

 chromosomes differ from one another in function, it might 

 be expected that the parts of the larva derived from one or 

 more of the first four blastomeres should sooner or later 

 show abnormalities. And this is what is actually found, as 



