1929] Bahcock-Clausen: Meiosis in Crepis 411 



Not much of the structure of the chromosomes has been shown, and 

 it is hard to be convinced that these big chromosomes really have 

 exchanged parts at their ends, although the possibility of such 

 exchange cannot be denied. 



^to*- 



MEIOSIS IN THE HYBRIDS 



1. Crepis aspera X C. hursifolia. — This hybrid was more exten- 

 sively studied than the other two, because living material was obtain- 

 able for new fixation in the combination of Carnoy's and Navashin's 

 fixatives in addition to the Carnoy-fixed material procured by IIol- 

 lingshead. 



The affinity between the chromosomes of the two species is not 

 strong. Although in some cases 4 pairs of chromosomes can be found 

 during the heterotypic division, more often only 3, 2, 1, or no pairs 

 at all are found, as will be seen from table 1. Plate 59, figures 19-22, 

 shows that spiral chromatids are seen within the chromosomes of the 

 hybrids just as they are seen within the chromosomes of the parents 

 during diaphase. As shown by figures 19-20, chiasmas can also be seen 

 in the hybrid, indicating that crossing over might be found if offspring 

 should be procured. Attention has been paid to the fact that an 

 unequal pair ought to be found in all pollen mother cells with 4 pairs. 

 This unequal pair, consisting of a small hursifolia and a larger aspera 

 chromosome, can be traced all through. According to unpublished 

 investigations of Ilollingshead the small hursifolia chromosome is the 

 one with a satellite. M. Navashin (1925, p. 107) observed that the 

 satellites were connected with the nucleolus. In accordance there- 

 with the imequal pair in C. aspera X hursifolia, when present as a 

 pair, is found near the nucleolus (pi. 59, figs. 19-22). Whether the 

 partner of the small hursifolia chromosome is the satellited Z>-chromo- 

 some from C. aspera (M. Navashin, 1927, fig. 4c) could not be settled, 

 as the satellites do not show in the diaphase, but very probably it is 

 that one because the pair is located at the nucleolus. 



When an unequal pair is present, its partners are usually located 

 end to end (pi. 59, figs. 19, 20, 22). This should indicate that no 

 crossing over had taken place. Still, in a few cases there might be 

 a possibility for a chiasma; thus in figure 21 the unequal pair seems 

 to be parasyndetic at one end. A question which would be interesting 

 to decide, is whether these two chromosomes unite at the satellited 

 end or not. The chromosomes might be homologous with respect to 

 this end. 



