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1905] MOTTIER—HETEROTYPIC CHROMOSOMES 175 
me now, though the statement is made with reserve, that the double 
nature of the spirem at this stage, referred to by BERGHs and ALLEN 
is due to the phenomenon just stated, and not to the approximation 
of two distinct spirems. 
Omitting details, the next step of importance is the clear and 
unmistakable manifestation of the longitudinal splitting of the 
chromatin spirem. In Podophyllum the segments of the spirem do 
not divaricate as in Lilium candidum, for example, but frequently 
parts of the two daughter spirems do separate for considerable 
stretches. The segments are more or less twisted about each other. 
Following this stage, the spirem shortens and thickens somewhat, and 
the longitudinal fission becomes less and less distinct, and finally 
almost every trace of the double nature of the thread disappears. 
The thread does not shorten nor thicken as rapidly, nor to the extent 
that it is usually supposed to shorten and thicken, before its trans- 
verse segmentation into chromosomes, and it is just at this point that 
the writer and many others have been led into error. The thread 
does, of course, shorten and thicken to some extent, and as a result 
its arrangement reaches its greatest regularity. This is the stage of 
the loose or hollow spirem so frequently observed. However, there 
is no well-marked regularity in the convolutions of the spirem through- 
out its entire length; some of its turns follow the nuclear periphery, 
While others traverse the interior. In the nuclear cavity the turns 
are often short and kinked. In sections including the whole nucleus, 
it is not possible to follow accurately the entire thread, but it seems 
that there are few or no free ends, and very rarely is any trace of a 
longitudinal split discernible. 
The stage of the loose and more regular spirem seems to persist 
for some time, as it is frequently met with in the preparations. The 
hext step in the prophase has been one of the stumbling-blocks of 
cytologists, and it is the one that the writer ascribed to poor fixation 
in his earlier studies. It may be true that this stage is difficult of 
fixation, and that, together with its short duration, has probably been 
the main reason for the failure to understand its true significance. 
J ust before the transverse segmentation of the spirem and the final 
differentiation into the chromosomes, the loose spirem loses the 
tegular arrangement it may have had and undergoes a contraction 
