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 development’. 
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 gonotokonts the homologous chromosomes being near 
together need not seek out one another; they are ready to form 
gemini. The next stage is their separation tq 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. Hicker and more recently by V. Grégoire and his 
pupil C. A. Allen, by the researches conducted in the Bonn Botanical Institute, and by 
A. and K. E. Schreiner. 
3 C. A. Allen, A. and K. HE. Schreiner, and Strasburger. 
4H. de Vries and Strasburger. 
