No. 556] DEVELOPMENT IN NICOTIAN A 



221 



Again, I have found that in some sacs most of the 

 archesporium has broken down, but some few cells seem 

 to have escaped destruction and matured. The meta- 

 phase, so far as I have observed, is not so likely to be dis- 

 turbed. An occasional premature splitting of the chromo- 

 somes takes place, increasing the number to be seen in 

 the polar view of the nuclear plate. These are rare, but 

 they have been observed in both the abnormal strain and 

 the abnormal segregates. This feature has been referred 

 to as a premature splitting, 23 but it may be interpreted 

 as an actual increase in number such as Wilson found in 

 Metapodius 24 Stevens in Diabrotica 25 and Strasburger 

 in Wikstroemia. 26 



In one pollen mother-cell, 51 chromosomes were clearly 

 distinguishable, but disintegration had already com- 

 menced. In another case 30 were counted, the mother- 

 cell appearing perfectly normal, although in the anther 

 containing it irregular divisions were taking place. The 

 nuclear metaphases in which such an increase in num- 

 ber can be seen are rare, but so far as I have observed, 

 and I have counted many nuclear plates ideal for such 

 work, they only occur in the abnormal or in the abnormal 

 segregates. The heterotypic anaphases of the abnormal 

 often show the chromosomes lagging or distributed pro- 

 miscuously over the spindle. In only one case so far have 

 I found irregular conditions in the telophase and this 

 only in the case of one mother-cell. Counting is unsatis- 

 factory in the anaphase of the first division, as the nu- 

 cleus is small and the chromosomes are many. 



Irregular divisions are present in the homotypic, but 

 most of the abnormalities occur during the heterotypic 

 mitosis. 



a Preparatory for the homotypic division. 



"Wilson, E. B., "Studies on Chromosomes. V. The Chromosomes of 

 Vetapodias, a Contribution to the Hypothesis of the Genetic Continuity of 

 Chromosomes/' Journ. of Exp. Zool, 6: 147-205, 1909, 1 plate and 13 text 



"Stevens, N. M., "Further Observations on Supernumerary Chromo- 

 somes, and Sex Eatios in Diabrotica sorer," Biol. Bull, 22: 231-238, figs. 



