AMEIOSIS 473 



First, we sometimes find that side by side with the regular 

 ameiosis, as we may describe it, of the functional cells, there is still 

 the original condition of pairing in the non-functional cells. Or 

 again if this pairing is suppressed it is found to be equally sup- 

 pressed when the chromosome number is doubled and every chromo- 

 some has an identical mate. Moreover, meiosis in the female cells 

 is liable to much greater irregularity than in those of any hybrid, 

 for occasionally, as in Artemisia, typical autopolyploid hybrid 

 behaviour is replaced by the highly specialised ameiosis. 



Secondly, in species of Eu-hieracium (Rosenberg, 1917, 1927) it 

 has been possible to trace stages in the development of the sup- 

 pression. Applying to these the results of our earlier analysis of 

 aberrations of meiosis (Ch. X) it is possible to define the conditions 

 of the development. 



In the first stage of " degeneration of the reduction division," 

 i.e., abortion of the first meiotic division, the Boreale type, we see 

 the ordinary symptoms of hybridity : pairing at pachytene but not 

 at metaphase ; the failure of metaphase pairing is due to the same 

 cause in these hybrids as in the fragments of non-hybrid species, an 

 insufficient length of chromosome paired at pachytene to lead to the 

 formation of a chiasma. But, in the last stage, Pseudoillyricum, 

 with suppression even of pachytene pairing and the substitution of 

 mitosis for meiosis, we have a phenomenon that is not characteristic 

 of hybrids, as such. Clearly a primary condition of meiosis is 

 wanting, and on the precocity theory this might happen in two 

 ways : either (i) the division of the chromosomes is relatively 

 advanced, or (ii) the beginning of prophase is relatively retarded, 

 so that at leptotene the chromosomes are already doubled and 

 therefore have no attraction for one another. That both these 

 aberrations are indeed conditions of various types of abnormality 

 found in meiosis is shown by the time discrepancies observed in 

 several apomictic and sexual genera. The irregular nuclei are either 

 advanced or retarded (Table 62). 



Semi-mitotic meiosis occurs in Hieracium at an earlier stage in 

 the development of the flower than the normal meiosis of sexual 

 species (Rosenberg, 1927). This general timing abnormality is 

 associated with a special one, characteristic of semi-mitotic meiosis 



