268 



REDUCTIOX OF THE CHROMOSOMES 



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

 investi<;ation than Liliu))i, I'^i til/aria, 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 ot 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, X-Q). The two limbs of these bodies do not, as might be 



A 



■>-'^^ 



V B 



C 







G \^ 



Fig;. 134. — Tlie seconrl maturation-division in flowering plants. [B. SXRASBURGER and 

 Mullii.K; the others from Moi iiKR.] 



A. Nucleus of secondary spermatocyte {Podophyllum). D. Prophase of second division 

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

 female. E. Metaphase of second division {Podophyllnm, male). G. Initial anaphase {Lilmvi, 

 female). CD. illustrate Mottier's earlier conclusions. ('. Second division {Lilium, male), with 

 chromosomes bent together so as to simulate a sjilit. A Slightly later stage [Eritillaria, 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 



