1929] 



Babcock-Clausen: Meiosis in Crepis 



413 



When only three pairs are present, the two univalent chromosomes 

 usually are a short and a long one (pi. 59, fig. 24), but at least in 

 one pollen mother cell with only three pairs, one of the pairs was 

 unquestionably unequal (fig. 25). This shows that the affinity between 

 the two members of the unequal pair is not much inferior to the 

 affinity between some of the other chromosomes and it also shows that, 

 when there is variation in u umber of bivalents, the chromosome pairs 

 do not always follow the same consecutive order as regards pairing. 

 The unequal pair shows much resemblance to an XY pair of sex 

 chromosomes (pi. 59, fig. 29). 



The small bursifolia chromosome can be recognized through heter- 

 otypic anaphase (pi. 59, fig. 30), homotypic metaphase (pi. 60, fig. 33), 

 and anaphase (pi. 60, fig. 34). In figure 31, showing a heterotypic 

 anaphase, it apparently is hesitating between the two poles and has 

 split longitudinally. Unfortunately the hybrid is almost completely 

 sterile. It sets only a few seeds by open pollination. Cytologically 

 it would offer some advantages for a combined cytogenetic study, as 

 the small hursifolia chromosomes can be followed through almost all 

 phases of meiosis. 



The most common case of pairing between the chromosomes in 

 heterotypic metaphase is 3 bivalents -f- 2 univalents (table 1). Even 

 if several univalents are present, the distribution of the chromosomes 

 to the two poles in many cases will be 4 + ^ (table 3). This gives 4 

 chromosomes in most of the homotypic daughter nuclei (table 4). 



TABLE 3 



Distribution of the Chromosomes as Counted from Heterotypic Anaphases 



AND Homotypic Metaphases in Crepis aspera X hursifolia Fi 



(3/1/4 means 3 and 4 to the two poles and one detached.) 



TABLE 4 



Number of Chromosomes in Homotypic Daughter Nuclei of 

 Crepis aspera X bursifolia Fi 



Number of chromosomes 



Number of nuclei. 



26 



Total 



38 



