Homotypic Nuclear Division 105 
distinguished, their correct interpretation was impossible. This was 
accomplished after long and laborious research, which has recently 
been carried out and with results which should, perhaps, be regarded 
as provisional. 
Soon after the new study of the nucleus began, investigators 
were struck by the fact that the course of nuclear division in the 
mother-cells, or more correctly in the grandmother-cells, of spores, 
pollen-grains, and embryo-sacs of the more highly organised plants 
and in the spermatozoids and eggs of the higher animals, exhibits 
similar phenomena, distinct from those which occur in the somatic 
cells, 
In the nuclei of all those cells which we may group together as 
gonotokonts! (i.e. cells concerned in reproduction) there are fewer 
chromosomes than in the adjacent body-cells (somatic cells). It was 
noticed. also that there is a peculiarity characteristic of the gono- 
tokonts, namely the occurrence of two nuclear divisions rapidly 
succeeding one another. It was afterwards recognised that in the 
first stage of nuclear division in the gonotokonts the chromosomes 
unite in pairs: it is these chromosome-pairs, and not the two longi- 
tudinal halves of single chromosomes, which form the nuclear plate 
in the equatorial plane of the nuclear spindle. It has been proposed. 
to call these pairs gemini?. In the course of this division the spindle- 
fibrillae attach themselves to the gemini, i.e. to entire chromosomes 
and direct them to the points where the new daughter-nuclei are 
formed, that is to those positions towards which the longitudinal 
halves of the chromosomes travel in ordinary nuclear divisions. It is 
clear that in this way the number of chromosomes which the daughter- 
nuclei contain, as the result of the first stage in division in the 
gonotokonts, will be reduced by one half, while in ordinary divisions 
the number of chromosomes always remains the same. The first 
stage in the division of the nucleus in the gonotokonts has therefore 
been termed the reduction division’. This stage in division deter- 
mines the conditions for the second division which rapidly ensues. 
Each of the paired chromosomes of the mother-nucleus has already, 
as in an ordinary nuclear division, completed the longitudinal fission, 
but in this case it is not succeeded by the immediate separation of 
the longitudinal halves and their allotment to different nuclei. Each 
chromosome, therefore, takes its two longitudinal halves into the 
same daughter-nucleus. Thus, in each daughter-nucleus the longi- 
tudinal halves of the chromosomes are present ready for the next 
1 At the suggestion of J. P. Lotsy in 1904. 
2 J. E. 8. Moore and A. L. Embleton, Proc. Roy, Soc. London, Vol. uxxvit. p. 555, 1906; 
V. Grégoire, 1907. 
3 In 1887 W. Flemming termed this the heterotypic form of nuclear division. 
