79 



across the spindle fibres transversely to the long axis of the latter 

 (Fig. 3 a). The loop is also generally much bent up at its ends from 

 the surface of the spindle and attached to the latter by its middle 

 point (Fig. 4a). It will thus be seen that such chromosomes are 

 exactly comparable to those with outwardly directed cornua described 

 in the early astral figure among Lilies. 



The loop now begins to be lengthened out in the direction of 

 the spindle poles in the manner represented in Fig. 5 a, so that the 

 two off-standing processes are left as a projecting wing on either side. 

 These wings constitute the well-known lateral thickenings on the loops 

 of the heterotype divisions (figured but not explained by Flemming 

 in his memoir already referred to), and when the chromosomes have 

 become further stretched along the spindle axis, the daughter elements 

 separate by a transverse split at these spots. There is thus formed 

 a "corpora quadrigemina" appearance like that which we have de- 

 scribed in the case of the separation of the daughter -V's in plants*). 



Although, as we have said, the evolution of the chromosome is 

 to some extent modified by the initial kind of split which it acquires, 

 it appears that in all cases (in Triton) the open ends of the ellipses 

 or loops close up before they divide. We are led to this conclusion 

 because we have not hitherto found any open loops after the astral 

 figure has been formed. 



As in Lilies, the chief types of variations are connected with the 

 manner in which the loop is attached in the first instance to the 

 spindle threads. In some cases the loop opens widely as in Fig. 6 a 

 and such chromosomes appear generally to have originally been open 

 at one end because it will be seen that even at this stage the up- 

 turned ends (Fig. 6 a) are not absolutely fused. Such chromosomes 

 remain expanded to the last and split eventually at or near the up- 

 turned hump and on the opposite side (Fig. 6 a, 7 a etc). 



It will be seen that this type of chromosome corresponds exactly 

 to that described by Miss Sargant in the case of Lilies*), but that 

 the order of its evolution as described by this author is here at any 

 rate reversed. The stage figured by her as final, but which is in no 



1) From what has already been said as to the origin of the "rings" 

 it will be clear that the apperently transverse fission of the chromosome 

 in reality is only the final stage of the initial longitudinal splitting in the 

 thread which occurs at an earlier stage in the history of the individual 

 chromosome. 



2) E. Saegant, Journal of the Royal Micr. Soc, June, Part 3, 

 1895, p. 283 - 287. 



