STOUT AND SUSA, CHROMOSOME IRREGULARITIES 9 



IRREGULARITIES IN THE DISTRIBUTION OF CHROMATIN IN THE FIRST 



DIVISION 



Perhaps the simplest phenomenon in the irregular distribution of 

 chromatin is the lagging of a single chromosome in the first meiotic divi- 

 sion (PL II, Figs. 19 and 20) or the isolation of a pair that has not dis- 

 joined. As already stated, this may occur when the number is normal 

 (12 diploid) and there has been complete pairing. A small accessory 

 nucleus may then be formed, as was described and figured by Stras- 

 burger (1882). There may be more than one lagging or stray unit. The 

 distribution in the first division may give three or more groups with 

 various numbers in each. A case of three groups with four chromosomes 

 (some already split or fragmenting) in each is shown in PL II, Fig. 21. 

 With an increase in the number of chromatin units and of chromosomes 

 during the early metaphases the distribution that follows becomes increas- 

 ingly irregular. 



IRREGULARITIES IN THE FORMATION OF CELL PLATES IN THE FIRST 

 MEIOTIC DIVISION 



Cell plates may or may not be formed at the close of the first division. 

 When a plate is fully developed, it results in two cells either with the 

 nuclei organized, permanently or merely for a time, or with the chroma- 

 tin units scattered. There appear, however, to be few cases where only 

 two spores are formed from one pollen mother cell, hence such condi- 

 tions as are shown in PL II, Fig. 27 are usually followed by the division 

 of at least one of the two cells. Where three or more nuclei are formed 

 after the first division and these become fully separated by cell plates, 

 further division evidently does not occur at least in the case of most of 

 the cells. Most instances of three microspores in a group and some of the 

 instances of four spores of irregular size are the products of only one reg- 

 ular division. In many cases the chromosomes pass on into the second 

 division without the formation or the persistence of definite cell plates 

 (PL II, Fig. 30 and PL III, Fig. 31). When the number of chromatin 

 units is large, as after fragmentation, the units may lie scattered through- 

 out, the cytoplasm (PL II, Fig. 24) and the organization of the nuclei is 

 delayed and irregular, the entire group passing on into the second divi- 

 sion without the formation of a definite nucleus. 



IRREGULARITIES IN THE DISTRIBUTION OF CHROMATIN DURING THE 

 SECOND DIVISION OF MEIGS IS 



In the second division or in the final stages preceding the formation of 

 the nuclei, the distribution of the chromosomes is most often irregular 



