Sexual Nuclei in Lilium Mar tag on. 469 
deviation from the ordinary type of karyokinesis, but its 
nature and extent are worth consideration. 
The characteristic features of the first division of the 
embryo-sac nucleus are (1) the long period of growth and 
development before the formation of the spirem thread and 
its division into chromosomes, and (2) certain peculiarities of 
form which characterize the chromosomes while separation 
of the segments is taking place. To this variation in shape of 
the chromosomes little importance need perhaps be attached. 
They are not alike when the spindle first appears. Each 
is formed of two segments much twisted on each other, 
but the number of twists, their tightness, and the disposition 
of the free ends vary in different chromosomes. The spindle 
fibres commonly attach themselves to one of the loops in the 
double chromosome, or sometimes to one end. It is clear 
that the two segments of a chromosome will not separate in 
the same way when they are pulled apart from the middle as 
when the loop where separation begins is near one end or the 
spindle fibres are fixed to the ends themselves. Besides this, 
it must be considered that the segments of each chromosome 
untwist from each other during the process of separation, and 
this of itself gives rise to much variation in shape, and often 
causes two adjacent chromosomes in the same nuclear plate 
to look very different (cf. Fig. 22 , a-c). The segments of 
a chromosome during karyokinesis of the vegetative type 
separate regularly from one end and on one uniform plan, 
probably because they are little if at all twisted on each 
other (Fig. 5), and do not appear until the chromosomes are 
in order on the spindle. Thus the peculiarities of the mature 
chromosomes in the first division 0/ the embryo-sac nucleus 
can be traced back to early stages in its development. 
The history of that development has already been told in 
detail. 
The nucleus passes through four phases — resting-stage, 
synapsis, spirem, and segmentation. It is interesting to 
enquire whether parallels can be found to any or all of these 
stages in the history of the vegetative nucleus. 
