256 INTERNAL FACTORS IV. i 



number of chromosomes o precisely the same shape and size. 

 In fertilization the two sets of chromosomes each. in half the 

 normal number are united. It is of course difficult to believe 

 that this extraordinary resemblance of the nuclei, while all the 

 other characters of the cells are unlike, is without significance. 



3. With regard to the third point a distinction must be made 

 between two hypotheses. The first is the individuality of the 

 chromatin, the second the individuality of the chromosomes. 



For the first, as we shall see, independent evidence exists, and 

 mitosis is certainly a mechanism admirably adapted to the 

 simultaneous division (or separation of already diyided halves) of 

 a large number of qualitatively distinct bodies. 



But the second hypothesis in no way follows from the first, 

 for the grouping of the unlike units may obviously be different 

 at each successive division without in the least impairing their 

 individuality. There is, indeed, evidence for the persistent in- 

 dividuality of the chromosomes in only one or two cases. 

 Boveri has noticed, in the segmenting egg of Ascaris, a constancy 

 in the position of the pockets of the nuclear membranes which 

 lodge the ends of the chromosomes, and a similarity in the 

 arrangement of the chromosomes in the nucleus in successive 

 nuclear divisions. Sutton again has observed that in Bracfiy- 

 ztola each chromosome forms a separate reticulum, situate in a 

 separate pocket of the nucleus. There are other cases in which 

 each chromosome forms its own vesicle (Echinoderm eggs). These 

 instances are, however, few and far between. There are so many 

 nuclei in which nothing can be observed of the chromosomes in 

 the resting stage, often there is nothing but a fine granular 

 mass, and a study of the early and end stages -of mitosis seems 

 to show that the chromosomes are precipitated from sohition in 

 the nuclear sap and redissolved when the division is over. 



In such a solution the chromatin elements would retain their 

 individuality, their qualitative differences, just as each one of 

 a number of crystals retains its distinctive properties in a mixed 

 solution and exhibits them when recrystallization occurs. In 

 such a sense, and such only, is it possible to speak in general of 

 the individuality of the chromatin. 



4. With regard to the diminution of the chromosomes in 



