NUCLEAR OSMOSIS AS A FACTOR IN MITOSIS. 153 
threads become longer and more radial in their distribution; they also become more 
numerous and more sharply defined. 
My interpretation of these kinoplasmic threads in Hedera is the same as that for the 
types mentioned above. These threads simply express lines of tension in the cyto- 
plasm—a tension caused by the increased cytoplasmic area, and which is sufficient to 
change the configuration of the reticulum, and the threads are therefore the passive 
results of nuclear osmotic changes. 
In fig. 37 we have a typical multipolar stage with the numerous chromosomes 
crowded together in the smal] remaining vacuole of karyolymph. Nearly all of the 
chromosomes are now in close touch with the nuclear membrane. The closing in of 
the membrane continues until it completely envelops each separate bivalent chromo- 
some in the manner above described for Disporum. The large number of chromosomes in 
Hedera, however, complicates matters, and we have the confused appearance represented 
in fig. 38. I have not the slightest doubt, however, that the fibrils become attached to the 
individual chromosomes in this manner. With the establishment of so many smaller 
osmotic systems there would naturally follow a final readjustment of the lines of tension 
expressed in the achromatic figure. This readjustment would be mainly controlled by 
the osmotic surfaces enveloping each chromosome. There was no indication whatever 
to suggest the bodily shifting and ultimate coalescence of the sheaves of the multi- 
polar figure. The establishment of the equatorial plate and the symmetrical bipolar 
arrangement of the lines of tension represented in the fibrils is evidently brought about 
in the manner already described above in the case of Disporwm and Gladiolus. 
ALLIUM. 
It is well known that spindle development in somatic cells is somewhat different 
from that occurring in spore mother-cells. This fact was established by NEmxc (1899) 
and others, who found that as the kinoplasmic weft becomes differentiated about the 
nuclear membrane, it takes the form of two conical-shaped caps which project from 
opposite sides of the nuclear cavity. These kinoplasmic projections, on account of 
their position, are commonly referred to as polar caps. The threads of which they 
are composed eventually become the main fibrils, and their apices become the poles of 
the spindle. So that in vegetative cells there is nothing in the nature of the multipolar 
arrangement which is so characteristic of the heterotype mitosis. It seems that the 
mitotic spindle in somatic cells is always bipolar from the beginning. 
Because of this striking difference between the somatic and heterotype mitosis, 
and in view of the new facts and interpretations recorded above in the cases of 
Disporum, Gladiolus, Yucca, and Hedera, I have considered it advisable to re-examine 
the nuclear changes leading to the formation of the achromatic spindle in the root tip 
of Alliwm cepa, a plant which has become a classic for nuclear study. 
The first indication of approaching mitosis is a decided enlargement of the nuclear 
TRANS. ROY. SOC. EDIN., VOL. XLVIII. PART I. (NO. 7). 24 
