110 Cell Structure in Relation to Heredity 



The fact that in nuclei containing chromosomes of various sizes, 

 the chromosomes which pair together in reduction-division are always 

 of equal size, constitutes a further and more important proof of their 

 qualitative difference. This is supported also by ingenious experi- 

 ments which led to an unequal distribution of chromosomes in the 

 products of division of a sea-urchin's egg, with the result that a 

 difference was induced in their further developments 



The recently discovered fact that in diploid nuclei the chromo- 

 somes are arranged in pairs affords additional evidence in favour of 

 the unequal value of the chromosomes. This is still more striking in 

 the case of chromosomes of different sizes. It has been shown that 

 in the first division-figure in the nucleus of the fertilised egg the 

 chromosomes of corresponding size form pairs. They appear with 

 this arrangement in all subsequent nuclear divisions in the diploid 

 generation. The longitudinal fissions of the chromosomes provide 

 for the unaltered preservation of this condition. In the reduction 

 nucleus of the gonotokouts the homologous chromosomes being near 

 together need not seek out one another ; they are ready to form 

 gemini. The next stage is their separation to the haploid daughter- 

 nuclei, which have resulted from the reduction process. 



Peculiar phenomena in the reduction nucleus accompany the 

 formation of gemini in both organic kingdoms^. Probably for the 

 purpose of entering into most intimate relation, the pairs are 

 stretched to long threads in which the chromomeres come to lie 

 opposite one another ^ It seems probable that these are homo- 

 logous chromomeres, and that the pairs afterwards unite for a short 

 time, so that an exchange of hereditary units is rendered possible*. 

 This cannot be actually seen, but certain facts of heredity point 

 to the conclusion that this occurs. It follows from these phenomena 

 that any exchange which may be effected must be one of homologous 

 carriers of hereditary units only. These units continue to form 

 exchangeable segments after they have undergone unequal changes ; 

 they then constitute allelotropic pairs. We may thus calculate what 

 sum of possible combinations the exchange of homologous hereditary 

 units between the pairing chromosomes provides for before the 

 reduction division and the subsequent distribution of paternal and 

 maternal chromosomes in the haploid daughter-nuclei. These nuclei 

 then transmit their characters to the sexual cells, the conjugation of 



1 Demonstrated by Th. Boveri in 1902. 



' This has been shown more particularly by the work of L. Guignard, M. Mottier, 

 J. B. Farmer, C. B. Wilson, V. Hacker and more recently by V. Gr6goire and bis 

 pupil C. A. Allen, by the researches conducted in the Bonn Botanical Institute, and by 

 A. and K. E. Schreiner. 



* C. A. Allen, A. and K. E. Schreiner, and Strasburger. 



* H. de Vriee and Strasburger. 



