268 



REDUCTION OF THE CHROMOSOMES 



in the pollen-mother-cells of Iris a much more favourable object of 

 investigation than Liliiim, Fritillaria, and the other forms on which 

 most of the work thus far has been done, and one in which the sec- 

 ond division takes place with "admirable clearness"; he also gives 

 interesting additional details of the first division in this and other 

 forms. In the first division the spireme splits lengthwise, and then 

 breaks into chromosomes, which assume the shape of a V, Y, or X 

 (Fig. 135, N-Q). The two limbs of these bodies do not, as might be 



Fig. 134. — The second maturation-division in flowering plants. [5. StraSBURGER and 

 MOTTIER; the others from MOTTIER.] 



A. Nucleus of secondary spermatocyte {Podophylhtm). B. Prophase of second division 

 {Lilium, male) with longitudinally divided chromatin-threads. E. Corresponding stage in the 

 female. F. Metaphase of second division {Fodophyllnm, male). G. Initial anaphase {Lilium, 

 female). CD. illustrate Mottier's earlier conclusions. C. Second division (/,///«/«, male), with 

 chromosomes bent together so as to simulate a split. D. Slightly later stage {Fritillaria, male), 

 showing stage supposed to result from breaking apart of the limbs of the U at point of flexure. 



supposed, represent sister-chromosomes (resulting from the longitu- 

 dinal division of the spireme) attached by one end or at the middle, 

 since each X, Y, or V is double, consisting of two similar superim- 

 posed halves. Belajeff, therefore, regards these figures as longitu- 

 dinally divided bivalent chromosomes, having the value of tetrads, 

 each limb being a longitudinally split single chromosome. The 

 double V's, Y's, and X's take up a position with the apex (or one end 

 of the X) attached to the spindle, and the longitudinal division in the 

 equatorial plane. The halves then progressively diverge from the 



