10 Proceedings of the Royal Irish Academy. 



Figs. 14, 15, 16, and 17 represent the equatorial aspect of a chromo- 

 some in the equatorial plate. Fig. 1 4 sho"ws the chromosome in the plate 

 before the processes have become drawn out along the spindle -fibres. In 

 15 the processes of the primary chromosome marked with circles are 

 shown connected with fibres both from the north pole and the south, pole. 

 The half marked with small circles is attached to the north polur 

 fibres, and that with the larger circles to the fibres of the south pole. In 

 the same manner the longitudinal halves of the dotted primary chromo- 

 some are connected with opposite poles. "When the splitting of the 

 cbromosomes is effected, the process continues, as is represented by 

 figs. 16 and 17. The half marked with large circles draws out between 

 the halves marked with small dots and small circles. These latter are 

 drawn towards the north pole, and form a V-shaped daughter chromo- 

 some, composed of half the dotted primary chromosome and half of the 

 primary chromosome marked with circles. The limbs of the V are 

 caused to diverge by the drawing out of the half marked with large 

 circles between them. The V formed simultaneously of the remaining 

 halves of the primary chromosomes is drawn to the south pole. Its 

 limbs are splayed by the fact that the half marked with small dots is 

 drawn out between them. Fig. 1 7 will make the fijial stage of the process 

 clear. Fig. 18 represents the general appearance of the two daughter 

 chromosomes leaving the equatorial and viewed in a radial direction 

 looking towards the centre. The ends of the limbs are often twisted 

 slightly round each other, as shown in this figure ; sometimes, how- 

 ever, they lie side by side. As the chromosomes draw apart from one 

 another the ends untwist, until finally, when just parting asunder, 

 they are vertically over one another in the position most usually 

 figured. But, again, it very often happens that while tbe twist of one 

 pair of limbs unravels, the other pair remain engaged, and so an 

 appearance such as is shown in fig. 7 is produced. This appearance is 

 also figured by Strasburgher and Mottier. ^ In the diasterthis retar- 

 dation manifests itself by the fact, that one limb of the V as it tends 

 towards the pole is drawn out and very attenuate towards the apex 

 (fig. 8), and lags behind the other. 



From the above description of the fonnation of the chromosomes, 

 and their division in the nuclear plate, it will be seen that the author 

 does not believe that this first mitosis of the spore-mother-cells is a 

 differential or " reducing " division. The two daughter nuclei receive 

 half of each primary chromosome, which is itself a segment of the 



iBer. d. deutscli. hot. Ges., 1897. Taf. xv., fig. 1. 



