6 ANNALS NEW YORK ACADEMY OF SCIENCES 



as six bivalents (PL I, Fig. 10) but usually there are various numbers of 

 bivalents. The following groupings of chromosomes have been found in 

 this stage: — 6 pairs; 5 pairs -4- 2 single; 4 pairs -4- 4 single; 3 pairs -4- 

 6 single; 2 pairs -f- 8 single; 1 pair -f- 10 single; and 12 single, which 

 are all the combinations of bivalents and univalents possible for a total 

 of 12 chromosomes. Nuclei with more than six chromatin masses are 

 frequent in cells that are in the stages of diakinesis and in many cases 

 the number is obviously due to the presence of univalents. Thus in PI. I, 

 Fig. 7 there are evidently four bivalents and four univalents, making a 

 total of eight chromatin masses involving a total of 12 chromosomes, 

 which is the normal diploid number. 



Irregularities in the Number of Chromatin Units in Diakinesis 

 and in the flrst meiotic division 



A study of numerous nuclei in diakinesis and in the early stages of 

 the first meiotic division reveals that many nuclei possess more than 12 

 masses or units of chromatin. Thus far the highest number of masses 

 observed during diakinesis is 24 (PL I, Fig. 9), which was found in an 

 aceto-carmine preparation. In this and in similar preparations many of 

 the masses present are obviously single, but there are also cases of several 

 masses more or less connected. In nuclei at this stage (PL I, Fig. 8) 

 certain of the masses present evidently comprise more than the equivalent 

 of two of the smaller masses. Thus the number of chromatin masses 

 present in a nucleus during diakinesis is frequently greater than 12 and 

 there are noticeable irregularities in their relative sizes and shapes. 



Possibly the number of chromosomes present in certain pollen mother 

 cells may sometimes be more than 12 due to an irregular distribution 

 in the somatic divisions that precede their formation, such as occurs in 

 the cells of the roots. But such conditions as arc shown in PL I, Figs. 7, 

 8 and 9, indicate that there are frequently failures in the reconstruction 

 of chromosomes after synapsis. The evidence is very positive that the 

 breaking up of chromosomes into fragments may and does occur during 

 diakinesis (PL I, Figs. 8 and 9). Later, during the early stages of the 

 first meiotic division, fragmentation of chromosomes is also frequent. 

 Thus in PI. I, Fig. 12 the six bivalents are in the earlier stages of frag- 

 mentation and segmentation and in PI. I, Fig. 16 all the bivalents and 

 univalents are in the process of breaking u}> into numerous pieces. Later, 

 after bhe fragments are more or less separated and there is a failure 

 or a delay in the organization of nuclei, the total number of pieces of 



