Reed . — The Nature of the Double Spireme in Allium Cepa. 277 
Mitosis in the Gametophyte. 
It has been shown above that A. Cepa has a longitudinally split spireme 
in its somatic nuclei. 
The nuclei of the pollen-grains have been examined, and it may 
be stated at once that they also show a similar type of spireme to that 
present in the somatic cells. 
PI. XIX, Fig. 29, shows the ‘ roof’ of a nucleus, and at this early stage 
it will be seen that the chromatin bands show well-marked splits. PL XIX, 
Fig. 30, shows a slightly older nucleus, and here again the longitudinal 
fission is apparent. The spireme which is ultimately formed is rather stout, 
and although it may not show the extensive splitting seen in PI. XVIII, 
Figs. 10, 11, and 12, of somatic nuclei, there can be no question that a 
split exists (PL XIX, Figs. 31, 32, and 33). It is during the last stages of 
prophase only that the longitudinal fission is obscured, and this is precisely 
the stage at which it is least obvious in the somatic nuclei. PL XIX, Fig. 34, 
however, shows a well-marked fission in one of the very stout and deeply 
staining chromosomes. 
The nucleus need not be followed through the subsequent phases of 
division since enough has been seen to clearly demonstrate that, apart 
from the chromosome number, there is no essential difference in the appear- 
ance of the spiremes in the somatic and gametophytic nuclei of A. Cepa. 
It may, however, be pointed out that, as the generative nucleus passes 
into the resting stage, it becomes very much elongated and eventually 
suggests a vermiform type of nucleus ; at the same time a well-marked 
vacuole is formed around it and the nucleus becomes suspended in this 
space by a spindle-shaped sheath of cytoplasm (PL XIX, Figs. 35 and 36). 
Both these nuclei also show distinct evidence of longitudinal fission in their 
chromatic elements. 
Discussion. 
It is not proposed to discuss fully the results of this investigation at 
the present stage; other phases in the nuclear history are being examined, 
so that a full discussion must necessarily be left to a later date, when 
all the facts may be brought into line. Certain points, however, may be 
emphasized here. 
It has been shown that both the somatic and the gametophytic nuclei 
have the double or longitudinally split type of spireme. This type of 
spireme in somatic cells is regarded by Overton ( 19 ), Sykes ( 23 ), and 
Takara ( 24 ) as representing a lateral approximation of the structures derived 
from the male and female gametes respectively, whilst Farmer and Digby 
(1 and 4 ) and Hof ( 9 ) regard it as representing a premature longitudinal 
fission which will effect the separation of the daughter chromosomes at the 
next division. 
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