THE MODIFICATIONS OF THE CHROMOSOME CYCLE 



59 



roses, etc., have been obtained by the vegetative propagation of somatic 

 mutations, which are often known as bud-sports. 



In other cases, reproduction, although not fully sexual, involves the 

 sexual organs or related structures. Such phenomena are known as 

 apomixis. In cases where the sexual cells (gametes) are concerned, we 

 speak of parthenogenesis; when the gametes fail and their functions are 

 taken over by other (gametophyte) cells we have apogamy, which occurs 

 in plants; while finally examples are known, also in plants, in which the 

 gametophyte is developed from cells other than the spore mother cell 

 and these are referred to as apospory. The nomenclature adopted here 

 is that proposed by Darlington. 



I. Parthenogenesis^ 



Parthenogenesis is of two kinds : diploid parthenogenesis, in which 

 the reduction is abortive and diploid eggs are formed which develop 



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fig. 22. Parthenogenesis in Animals. — Two diploid parthenogenetic life cycles 

 are shown on the left, the aphid with a succession of females reproducing by 

 diploid parthenogenetic eggs, the Phylloxeran with only two such females before 

 sexual forms are again produced. On the right are three cycles involving haploid 

 parthenogenesis. In the rotifers there is a series of diploid parthenogenetic 

 females, in the gall flies one, in the bees none. In all cases the males are haploid, 

 developed from unfertilized eggs. (From Wilson.) 



without fertilization, and haploid parthenogenesis in which normally 

 reduced eggs develop without fertilization. Either of these forms may 

 be found constantly in a species (obligatory) or only under certain 

 conditions (facultative). 



^ General references: Ankel 1927, 1929, Vandel 1931, For genetics of Protozoa 

 see Jennings 1930, Raifel 1932, Sonnebern and Lynch 1934. 



