STOUT AND SUSA, CHROMOSOME IRREGULARITIES 11 



division and <>. or close to that number, for each of four spores when 

 that number develops from a single pollen mother cell. 



The fate of these X-shaped chromosomes or cohering pairs of daugh- 

 ter chromosomes is a matter of interest. That they may remain in this 

 condition and pass into the primary nucleus of spores is indicated in 

 Figs. 33 and 35. But thus far in the mitosis of the primary nucleus of 

 microspores only rod-shaped chromosomes have been noted (PI. Ill, Figs. 

 36 and 37), which may indicate that nuclei receiving the X-shaped chro- 

 mosomes do not reach this more advanced stage of development, or that 

 the two pairs of the X become separated, or that they combine to make 

 a single rod. The shapes of chromatin masses may be still more abnor- 

 mal in that they may become much lobed or even almost amoeboid in 

 shape (PI. II, Fig. 24). 



THE STRETCHING OF CHROMATIN MATERIAL BETWEEN NUCLEI 



The stretching or stringing out of a single chromatin unit, as de- 

 scribed by Juel (1897), is frequently to be seen during both the first and 

 the second division, and this may continue or persist even after the reor- 

 ganization of the nuclei. In PI. II, Fig. 22, which is a telophase of the 

 first meiotic division, one chromatin unit is stretched between two daugh- 

 ter nuclei with a part in each. Quite the same condition is shown in 

 PI. Ill, Fig. 33 after the nuclei resulting from the second division are 

 fully formed. Evidently the two ends of a single chromosome may be 

 included in two different nuclei, or two daughter chromosomes which 

 have remained attached to each other are thus distributed. 



THE MICROSPORES AND MACROSPORES 



The Number of Microspores 



The number of microspores that may develop from a single pollen 

 mother cell has been especially studied in fresh material by using the 

 aceto-carmine method. In only one case has but one large cell been 

 found to be derived from a pollen mother cell. In the cytological prepa- 

 rations of meiosis the chromatin masses frequently lie scattered within 

 one cell with little indication of spindle fibres, but evidently divisions 

 which give two or more spores occur sooner or later. The number of 

 spores formed from a single pollen mother cell and their relative sizes, 

 with the frequency for each in the 624 cases counted, are as follows: — 



