152 Mr Gregory, The Abortive Development 



of the same size would contain pollen-mother-cells of a much later 

 stage, or even young pollen grains. 



The various forms which the irregularities take differ from one 

 another only in degree, so that although the plants fall roughly 

 into two groups — those in which chromosomes are formed and 

 those in which complete abortion takes place at an earlier stage — 

 intermediate conditions are frequently found. 



In the types more nearly approaching the normal form, the 

 nucleus of the pollen-mother-cell passes into the spireme stage, 

 the thread undergoes a longitudinal fission, and by its contraction 

 towards one side of the nucleus a series of irregular loops is formed 

 (Figs. 2, 5— 8). 



The formation of the chromosomes through the segmentation 

 of the looped spireme takes place in the normal way. Fig. 7 

 illustrates a cell in which only very slight irregularity is present ; 

 the chromosomes, however, possess a very much more ragged 

 outline than do those of normal pollen-mother-cells. It is perhaps 

 worth mentioning that in the cells of the loculus adjoining that in 

 which this cell occurred, the abnormality was much more pro- 

 nounced and the formation of chromosomes completely failed (cf. 

 p. 153). It will be seen that the chromosomes in this early stage 

 give the impression of being of different lengths; later they 

 become much shortened, and in metaphase no distinct differences 

 in size can be recognized (Figs. 19, 20). 



In other cases in which the attempted formation of chromo- 

 somes is successful there appears to be a great deficiency of 

 chromatin ; the chromosomes of the abnormal pollen-mother-cells 

 staining very feebly as compared with those of the surrounding 

 (normal) somatic cells (Fig. 6). 



The great irregularity of the spindle of the heterotype division 

 is a very marked feature in these plants ; and in addition to 

 irregularities in the direction taken by the spindle fibres them- 

 selves, the chromosomes are irregularly distributed upon the 

 spindle (Figs. 19, 20). No case has been observed of the regular 

 arrangement of the chromosomes in an equatorial plate at meta- 

 phase, such as obtains in the fertile plants. 



Although many anthers have been examined which, by com- 

 parison with the fertile plants, were expected to show mature 

 pollen grains (if any were developed), no stages later than that 

 just described have as yet been obtained. 



Cases, such as those described above, are comparatively rare. 

 They are characterized by the definite formation of chromosomes 

 and the development of a spindle, the abnormality being confined 

 to the irregularity and asymmetry of these processes. 



Much more commonly the abortion of the pollen-mother-cells 

 is complete and only the early stages of prophase are passed 



