STOUT AND SUSA, CHROMOSOME IRREGULARITIES 7 



chromatin may be as high as 50 or more (PL II, Fig. 24) and some of 

 these may be very small. 



During the first meiotic division an increase in the number of 

 chromosomes may and often does occur by the division (longitudinal 

 splitting) of univalents, such as regularly occurs in somatic divisions 

 and in the second meiotic division. In the equatorial plate of the first 

 division the univalents are often smaller than the bivalents, even when 

 the univalents are in the process of dividing. This is well shown in PL I, 

 Figs. 13 and 14, which are of two pollen mother cells lying side by side 

 in the same anther with the nuclei of both in the equatorial plate of the 

 first division. In PL I, Fig. 14 there are 12 univalents and each is di- 

 viding. In PL I, Fig. 13 there are 5 bivalents and 2 univalents. A 

 slightly later stage is shown in PL II, Fig. 17, with 12 chromatin masses 

 passing to each pole. Most of these appear to be double due to the begin- 

 ning of a second splitting. Thus from 12 univalents it is possible that a 

 total of 48 chromosomes may develop by two successive divisions. With 

 complete pairing and no fragmentation there is usually a reduction divi- 

 sion, although irregular distribution and non-disjunction may follow. 

 With increasing numbers of univalents the first division becomes more 

 and more like that of a somatic cell. When all 12 chromosomes are uni- 

 valents and there is division of each in the first division (PL I, Fig. 14), 

 a reduction of chromosomes is omitted and the division is identical with 

 that in somatic mitosis. Certain of the figures given by Juel (1897) 

 indicate that he had before him such a condition. It seems probable 

 that it was from observing such figures that Strasburger (1900) was 

 led to decide that the normal diploid number of chromosomes for plants 

 of II. fulva (the clon Europa) is 24. 



To summarize : during diakinesis and the first meiotic division marked 

 irregularities frequently develop in respect to the number of chromatin 

 units. Through the division of univalents the number may range from 

 the normal 12 to as many as 24. When there is also fragmentation, the 

 number of chromatin pieces may be greater. As a result the numbers 

 in the metaphases of the first division are very often more than 12 ; they 

 rather frequently rise to 24, and a total of more than 30 is to be found. 

 Further chromosome division and fragmentation in the second meiotic 

 division may again increase the number of chromatin units, although the 

 later splitting of chromosome units is very often not completed, at least 

 when the number has already been increased and especially when the 

 increase has been by the splitting of univalents. 



