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Allen. — Nuclear Division in the 
ingly single spirem gradually distributes itself more uniformly throughout 
the nuclear cavity. An important difference is manifest between the 
arrangement of the spirem at this time and its arrangement in the prophases 
of previous divisions. In the earlier divisions the arrangement of the 
spirem displayed a distinct polarity, and was always closely similar to 
its arrangement in the preceding anaphases. This is not the case, so far as 
my observation goes, at any stage in the prophases of the heterotypic 
division. It is true that, as already noted, some authors have found a 
certain polarity in the arrangement of the chromosomes after segmentation, 
and Strasburger has noted a regularity in the course of the spirem of 
Tradescantia ; and it is quite possible that in all cases some order prevails 
in the course of the unsegmented thread which has not been detected. 
But such polarity as may exist, at least in the case of the lily, is certainly 
very much less striking and characteristic than in the somatic divisions. 
It seems that the transformations undergone by the nuclear material during 
synapsis have resulted in the obliteration of nearly or quite all traces of its 
former symmetrical arrangement. 
The condition of apparently complete fusion persists, as has been said, 
for a long time, certainly for several days. There then occurs a longitudinal 
splitting of the spirem, which is preceded by a fission of each chromo- 
mere, and which to all appearances is similar to the splitting that occurs 
in the prophases of a somatic mitosis. This longitudinal splitting, if we 
were ignorant of the processes preceding, would naturally seem to be 
preparatory to an ‘ equation division ’ ; and so it has been interpreted 
by most of those who have observed it in the prophases of the heterotypic 
division, either in animals or plants. But, on the other hand, the figures 
produced by this splitting are so closely similar to those which were 
observed in the fusion that produced the single spirem as to suggest that 
possibly this splitting is really the separation of the two threads which 
have been for some time in very close combination. This view would make 
of the splitting a preparation for a separation of the idioplasms which have 
existed together throughout the life of the sexually-produced individual. 
The question of the nature of this splitting, and therefore of the heterotypic 
mitosis, depends upon what has occurred within each individual chromomere 
since the time of its formation from two separate bodies ; and this problem 
is as yet beyond the reach of observation. One a priori argument, however, 
is of some weight in this connexion — namely, that it is hardly conceivable 
that the fusion of the threads, with all of the elaborate adjustments that it 
involves, should have occurred if they are afterward to separate without 
any sort of interaction or inter-relation between them or their constituent 
parts. Experimental data, to which I shall refer on a later page, suggest 
more definite conceptions of the processes involved in the fusion and 
subsequent splitting of the spirem. 
