396 BREAKDOWN OF GENETIC SYSTEMS 



The commonest type of breakdown in mitosis is the failure of 

 two nuclei to be formed at anaphase or their fusion at telophase 

 to give a fusion nucleus with double the ordinary number of chromo- 

 somes. This abnormality has been described in relation to the 

 origin of polyploidy (Ch. III). We have seen that it is conditioned 

 by an abnormal environment, by injury, and so forth, as a rule. 

 It is also often conditioned in its frequency by the genotype, some 

 species and races being particularly liable to produce polyploid 

 cells, either generally or in certain tissues. We have also seen that 

 this " syndiploidy " is particularly frequent in the part of the life 

 cycle approaching meiosis. It is at this stage also that its genotypic 

 conditioning becomes most obvious (e.g., in Datura). It is then 

 often accompanied by other irregularities of mitosis. In Drosophila 

 simulans one of these affecting the premeiotic divisions in the ovary 

 as well as the cleavage divisions in the egg, has been referred to the 

 action of a particular gene (Sturtevant, 1929). 



The reverse effect of loss or breakage of particular chromosomes 

 is determined by particular genes in Drosophila, but the mechanism 

 seems to be structural, since only the chromosomes carrying the 

 genes in question are affected (Bridges, 1925 h ; Stern, 1927). 



(ii) Pollen Grains and Spermatids. Of the genotypically controlled 

 abnormalities of mitosis, the most significant, the permanent and 

 polyploid prophase in salivary glands, has already been described. 

 Here, of course, we are dealing with nuclei which will never divide 

 again. In pollen grains, on the other hand, abnormalities involve 

 sterility and are therefore limited to aberrant conditions or geno- 

 types. Abnormalities of development are frequent in them, such 

 as the embryo-sac-like pollen grain in Hyacinthus (Stow, 1930 et 

 alii), but these do not involve abnormalities of mitosis. 



A minor abnormality of mitosis is found in species of Fritillaria 

 and Tulipa (D., 1936 ; Upcott, 1936). Here in certain pollen- 

 grains a group of correlated differences in behaviour is found. 

 First, the chromosomes are more condensed than usual. Secondly, 

 their centromeres do not synchronise in division, i.e., in beginning 

 the anaphase separation ; the chromosomes do not lie on a flat 

 plate, but are irregular both in their distribution and orientation. 

 Thirdly, their distribution is more confined to the periphery of the 



