THE CELL AND ITS CONSTITUENT STRUCTURES. 159 



the same phenomenon has been also observed in living 

 pollen mother cells. ^ But whether the appearance is con- 

 nected with the process of reduction, or whether it is to be 

 ascribed to some interaction between the linin and some of 

 the other constituents of the nucleus, it would be unsafe to 

 state definitely at present. 



The fifth point lies in the early period at which the 

 longitudinal fission of the linin filament becomes apparent, 

 as compared with the ordinary dividing nuclei of the 

 somatic cells. 



The sixth point is the most important of all, namely, that 

 as soon as the number of the developing chromosomes 

 which are about to take part in this mitosis can be counted 

 they are seen to be only one half the number visible at the 

 corresponding stage of the previous nuclear generation, and 

 this number is adhered to in the succeeding nuclear genera- 

 tions until, in the act of fertilisation, it again becomes 

 doubled. Thus, however, we may finally explain it : the 

 reduction in the number of the isolated chromosomes is 

 seen to have been somehow or other brought about during 

 the resting stage of the nucleus, or at any rate in the very 

 early stages of mitosis. And this statement of the matter 

 has been accepted with more or less reservation by most 

 Botanists and Zoologists. Nevertheless we cannot blind 

 ourselves to the fact that certain difficulties do exist in the 

 way of its general application to cover all cases ; some of 

 these difficulties are matters of observation, others are ques- 

 tions of theory. 



Finally, the seventh point lies in the very remarkable 

 form assumed by the mature chromosome ; it often appears 

 as a closed ring which comes to be along the spindle, ulti- 

 mately splitting across the middle. A careful study ot the 

 earlier sta^res leads however to the conclusion that we are 

 nevertheless really dealing with a true longitudinal fission 

 of the original chromosome rudiment, only the mutual rela- 

 tionship of the parts has been obscured by developmental 



^ E. Sargant, " Tlie J'ormation of the Sexual Nuclei in Lilium 

 Martagon" Annals of Bota?iy, vol. x., 1896. 



