456 



BREAKDOWN OF GENETIC SYSTEMS 



mitosis and gives a dyad. This type of division is often found 

 together with the Icevigatum type, for variations always occur in the 

 same individual, as they do in all other irregular chromosome 

 behaviour (Rosenberg, 1927). All such variation may be explained 

 by the developmental variations having a certain threshold value 



Fig. 132. — Meiosis in pollen mother-cells of H. pseudoillyricum 

 (^^x — 27). A-C, early prophase. D, contraction replacing 

 first division. E-G, renewed prophase of second division. 

 H-O, second division which gives dyad of pollen-grains with 

 the unreduced chromosome number. (After Rosenberg, 1927.) 



in the effect on the general course of meiosis in a sexually fertile 

 organism (they can be detected only by their effect on chiasma 

 frequency) , while in hybrids and organisms with genetically abnormal 

 meiosis there is no such threshold value. 



A parallelism between pollen and embryo-sac mother-cell 

 behaviour is by no means universal however. For example, 

 Antennaria, Poa serotina and Hieracium umbellatum, all of which 



