712 Proceedings of the Royal Irish Academy. 



inner end and passes outwards to the periplieral end of tlie ckromosome 

 and causes it to assume the characteristic humped form, and later the 

 T- shape. In the latter stage the horizontal bar of the T is represented 

 by the inner ends of the dividing chromosomes which part asunder 

 from one another and bend up directly to the opposite poles, the vertical 

 bar corresponds to the still united peripheral portions of the chromosome. 

 Often before the inner ends are bent up towards the poles a more 

 transparent line may be seen dividing the chromosome horizontally and 

 along which the horizontal fission will take place. 



Judging, then, from the appearance of the chromosomes from these 

 two aspects — the polax and equatorial — we may conclude that each 

 chromosome at the beginning of the formation of the nuclear plate 

 is composed either of four short straight rods with their axes lying 

 more or less exactly along the radii of the equatorial circle (quadrate 

 chromosomes), or they may be united one with another at their inner 

 ends in such a manner that their four free ends lie towards the peri- 

 phery, while the angle included by the united ends is towards the 

 centre (triangular chromosomes). In some cases the peripheral ends of 

 each come into contact with one another, and so the annular chromo- 

 some, composed of two annular halves in apposition, is formed. In 

 fine, whether the chromosome in the equatorial plate is triangular, 

 quadrate, or annular, it is divided more or less completely into four 

 rod-shaped bodies by cleavages taking place in the vertical and 

 horizontal plane ; more strictly speaking, the division in the vertical 

 plane represents an incomplete fusion. Prom Strasburger's " Practical 

 Botany," chap. 32, and Guignard's figures {loc. ceY., figs. 13 and 14), 

 it is evident that they also have observed this structui'e in the 

 chi'omosomes of the equatorial plate in the pollen mother-cells, 

 since they figure the chromosomes as double when seen from the 

 polar as well as from the equatorial aspect, but so far as I am aware 

 they have not fully described it. Parmer (Plora, 1895) also describes 

 and figures the four free ends directed towards the periphery, but these 

 are in the case of L. longiflorum only occasionally to be made out 

 with distinctness. More recently Strasburger ( " Karyokinetische 

 Probleme," Jahrb. f. wiss. Bot. 1895) describes this process as a 

 double longitudinal fission taking place to prepare the chromosomes 

 for this as well as the following division in which he denies that 

 longitudinal fission of the chromosomes takes place. It is to be noticed 

 that he has not, in this last-mentioned work, re-examined the origin 

 of the double thread, but bases his belief of its origin by fission on 

 previous descriptions. The observations already described are agaiust 



