VARIATION IN THE SPINDLE 541 



the other, as in male Hymenoptera (Katayama, 1935, in haploid 

 Triticum monococcum, and Fig. 127). Similarly it often happens 

 that a lagging chromosome at first anaphase has a cell-wall formed 

 around it within which it fonns a nucleus (D., 1929 b). 



When, however, an acentric fragment is left on the plate it never 

 forms a cell-wall. Nor do chromatid bridges affect the formation 

 of a cell-wall, which merely cuts through them (D., 1929 b). It 

 therefore seems that, while the bodies of the chromosomes are rela- 

 tively negUgible, centrosomes, spindle poles and centromeres are of 

 equivalent importance in determining cell-wall formation and hence, 

 we must infer, in their action on the spindle. 



(c) The Spindle in Movement. The uniformity that we have seen 

 in the structure and function of the spindle in no way corresponds 

 to a uniformity in its method of origin. In this respect it is 

 extremely variable and we have to find out how far its variations 

 are mechanically significant. In the first place there is the differ- 

 ence between those spindles which develop between two daughter 

 centrosomes while they are still close together and those which arise 

 from two poles on opposite sides of the nucleus. The first charac- 

 teristically gives a central spindle, as found in Salamandra and 

 Aggregata, where the long chromosomes lie with their centromeres 

 on the edge of the plate and their bodies in the cytoplasm (Belar, 

 1926). The other method of origin characteristically gives a spindle 

 with an even distribution of the chromosomes on the plate, as in 

 the Allium type of the higher plants. Variations in this behaviour 

 are found in organisms of both types. The early cleavage divisions 

 in Ambly stoma are of the Allium type, and sometimes, as we saw in 

 pollen grains of Fritillaria and Ttdipa, the chromosomes lie entirely 

 on the edge of the plate. Here the abnormality goes with an abnor- 

 mality in the degree of contraction of the chromosomes and in the 

 timing of the division of the centromeres. The peripheral distribu- 

 tion is what would be expected with a higher repulsion from 

 the poles, due to a higher charge or a smaller spindle, and variation 

 in this direction is the commonest departure from the even 

 distribution given by Meyer's floating magnets {cf. Kuwada, 1928). 

 The significance of a relationship of distribution and timing 

 abnormalities will be seen later. 



