206 BOTANY 



probably in each daughter nucleus one univalent will be missing, 

 whilst another is duplicated. Irregularities in segregation may be 

 the result of non-disjunction of a pair of chromosomes, the 

 daughter nuclei thus receiving different numbers of chromosomes. 

 Such segregation also results in the exchange of homologues 

 between the two daughter nuclei. Usually, in non-disjunction, 

 six chromosomes pass to one pole whilst eight pass to the other ; 

 occasionally, however, five-nine divisions are observed, and 

 presumably they arise in a similar manner. The daughter nuclei 

 formed as a result of such uneven segregation at meiosis continue 

 their development in the normal way. Undoubtedly pollen grains 

 with eight chromosomes are formed and function in fertilisation 

 with the ultimate production of trisomic mutants. Presumably 

 those with six chromosomes do not function, as no thirteen 

 chromosome mutants are known. Those irregular segregations 

 which do not result in an uneven distribution of chromosomes may 

 still be of considerable importance genetically. Double non- 

 disjunction may occur, homologues passing towards the same 

 pole in each of two cases. 



The significance of these results, however, cannot be properly 

 interpreted until more is known of the process of meiosis in the 

 megaspore mother-cell. Till the recent publication by Gates and 

 Sheffield, it was not known whether the process of meiosis was 

 precisely the same as that which occurs in the pollen mother-cells. 

 The only previous publication bearing on this aspect of the 

 problem was by Davis on CE. biennis, who described strings of 

 chromosomes in the megaspore mother-cell. It would thus 

 appear from this work that regular groups of linked chromosomes 

 are also formed in the megaspore mother-cell as well as in the 

 pollen mother-cells. 



Gates and Sheffield have investigated megaspore formation in 

 CE. ruhricalyx in connection with linkage, which is so evident in the 

 pollen mother-cells in this genus. In the heterotypic division of 

 this form the reticulum becomes coarser and the nucleoli lose 

 their vacuolate appearance, but budding of the nucleolus was not 

 observed. An endonucleolus could, however, be seen. The 

 reticulum contracted away from the nuclear membrane and 



