6 Proceedings of the Royal Irish Academy. 



The strepsinema condition having arisen as we have seen from the 

 twisting together of the dolichonematoiis thread, the nucleus enters on 

 the next stage in which the double thread is broken into a number oi 

 chromosomes, each composed of two portions twisted and looped 

 together, as is shown in figs. 3 and 4. The chromosomes are of 

 very different lengths ; in some the component chromatin threads 

 take two or more complete turns round one another, in others 

 a quarter revolution is all that each short thread takes round 

 its neighbour (fig. 4). Towards the end of this stage the chromo- 

 somes are short and bulky, and the direction of the twist of the 

 component parts is easily studied. It is remarkable that in the same 

 nucleus the twist is not always in the same direction, but we may find 

 a chromosome with a right-handed twist lying side by side one with, 

 a left-handed twist. "While the chromosomes are becoming shorter 

 changes are noticeable in them ; the chromomeres lose their individu- 

 ality, and all merge in the densely staining chromosome, the twisting 

 in each chromosome is reduced to one complete turn or less, and all 

 signs of the longitudinal split, real or apparent, in the components 

 portions disappear. Little or no diminution in distinctness of the 

 two twisted portions is observed. 



With regard to the nuclear plate stage, some discussion^ has been 

 raised as to whether the two portions of the chromosomes lie in the 

 equatorial or in meridional planes ; as a matter of fact careful observa- 

 tion shows that in the same nuclear plate the peripheral ends of 

 different chromosomes lie indifferently in either of these planes or in 

 planes inclined to them. The fact that the portions are twisted on 

 each other makes it impossible to assert that the portions themselves 

 lie in any one plane. 



I formerly believed that of the looped chromosomes the closed end 

 was regularly directed towards the axis of the cell and the free ends 

 directed towards the periphery.- Having found several undoubted 

 instances of the looped end being directed outwards, I am compelled to 

 consider that it is a matter of accident in which, direction in the 

 nuclear plate the chromosomes lie. The same nuclear plate actually 

 sometimes shows chromosomes in the two positions (fig. 5). 



The manner of attachment of the chromosomes to the fibres of the 

 achromatic spindle, and the method of formation of the two V-shaped 

 daughter chromosones from the thick twisted chromosomes of the 

 equatorial plate are matters of dispute. Several authors believe that 



^ Gregoire, he. eit. • Dixon, loc. cit. ; Schaffner, loc. cit. 



