UPSET OF MITOSIS 397 



spindle in the way that is characteristic of animals having the 

 " hollow spindle." These differences can probably be related best 

 by supposing that there is an irregularity in the timing relationship 

 of the spindle and of the chromosomes which as we shall see later 

 are independent but usually co-ordinated factors in metaphase and 

 anaphase movements. 



(iii) Polymitosis. Meiosis may be more or less effectively 

 replaced by mitosis under special genetic conditions — less effectively 

 in adventitious mutants such as asynaptic maize, more effectively 

 in races with adaptive parthenogenesis (v. Ch. XI). It is equally 

 to be supposed that mitosis might ander special genetic conditions 

 be replaced by meiosis. This is possibly so at an early stage in the 

 testis of Icerya and in the antheridia in the diploid gametophytes 

 of some aposporous ferns (v. Chs. IX and XI) where the normal 

 occurrence of meiosis is also omitted, in compensation. The 

 simple intercalation of meiosis is, of course, bound to be destructive 

 of the normal sexual cycle, and cannot therefore be expected save 

 in abnormal conditions or as an adventitious mutation. In this 

 way it seems to have been discovered in the form of supernumerary 

 divisions in the pollen grains of races of Zea Mays by Beadle (1931). 

 This property of having " polymitosis " is inherited as a mendelian 

 recessive character. 



In an abnormal plant the four nuclei of the pollen tetrad formed 

 by a normal meiosis enter into a prophase condition soon after the 

 end of the second division. The chromosomes are still single at the 

 early stages but are certainly sometimes double at metaphase, when 

 it is found that occasional pairs are formed, characteristic bivalents 

 with single interstitial and terminal chiasmata (Plate X). At 

 metaphase the chromosomes have the extra contraction found at 

 meiosis ; they sometimes form a metaphase plate, but they are 

 usually scattered over the spindle and are distributed at random to 

 the two poles at anaphase. The chromosomes have not been seen 

 to divide at their centromeres, but probably do so occasionally. 

 Their behaviour is therefore the same as that of univalents at 

 meiosis. This mitosis is followed immediately by two or three 

 more, and cell-walls are formed between the products. Thus 10 

 chromosomes or a few more are distributed to 8, 16 or 32 cells, 



