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 1 (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 2 . 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 3 . 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. S. Moore and A. L. Embleton, Proc. Roy. Soc. London, Vol. LXXVII. p. 555, 1906; 

 V. Gregoire, 1907. 



3 In 1887 W. Flemming termed this the heterotypic form of nuclear division. 



