26 DUBLIN 



The chromosomes are now drawn more uniformly into the 

 equatorial plate, but there is a considerable amount of variation 

 in the development of the spindles at this stage. Thus, while 

 as in Fig. 88 the nuclear membrane still persists and the spindle 

 has not yet reached its maximum length, in Fig. 89, in contrast, 

 although the chromosomes are not as fully drawn into the equa- 

 tor, yet the spindle is in its final form. It is such stages as 

 these that make possible a confusion with the later anaphases. 

 The shortened and bent bars, distributed on both sides of the 

 center, often give the appearance of dyads which might have 

 arisen by the cross division of the rings. Careful count of the 

 total number of chromosomes has, however, alwa)^s given the 

 reduced number, and this, together with the fact that the spin- 

 dles have not as yet moved to the periphery, has convinced me 

 that the bars are not dyads but the same structures observed in 

 the immediately preceding prophases. It is at this point, too, 

 that the cross-chromosomes make their appearance. These, in 

 some instances, show an open space at the center. The arms 

 are of unequal size, and the larger lies in the long axis of the 

 spindles. These considerations would tend to show that these 

 figures had arisen, like those described by Griffin, '99, in TJialas- 

 sema and ZirpJicea, from double bars which had extended out 

 from the middle or more probably as Conklin, '02, suggests, in 

 his work on Crepidiila, by the flowing out of the substance of 

 the rings. In any case, it is hardly possible that the arms rep- 

 resent dyads which have prematurely separated and then ro- 

 tated on each other. 



The spindle, as a whole, moves toward the periphery and the 

 metaphase of the first maturation division is attained (Figs. 90— 

 94). The chromosomes elongate at the same time, and the 

 ring chromosomes of various form are now also observed as 

 thick rods in which the longitudinal split is more or less dis- 

 tinct. In spite of the former diversity, there is at this stage a 

 remarkable uniformity in the appearance of the chromosomes. 

 We may also, at this point, decide upon the nature of this di- 

 vision. It will be necessary, for this purpose, to review briefly 

 the history of the chromosomes (cf. Text-fig. I, p. 13). It was 



