28 Overton. — On the Organization of the Nuclei in the 
earlier results a detailed account would be a needless repetition. In this 
plant the chromosomes become arranged about the periphery of the equa- 
torial plate and have the spindle fibres attached at their inner ends. The 
substance of the plastic chromosomes is often drawn out into a small point 
at the place of their attachment to the spindle fibres. In case the attach- 
ment is at one end, the separation of the two component chromosomes is 
usually from within outward (Figs. 25, 26, 27 a, and 27 c, PI. I). 
At the time the portions of the bivalent chromosomes are separating, 
the longitudinal split in each part becomes very apparent (Figs. 27 b and 
2 7 <2, PL I). These figures show very clearly that the chromosomes are 
four-parted (tetrads). Allen (’ 05 ) believes the appearance of this split to 
be very sudden in Lilium canadense. In Fig. 22^, Pl. I, which represents 
a chromosome from diakinesis. the two univalent chromosomes are 
widely separated except at the ends. In the portion at the right of this 
figure a split is seen to be just appearing, or at least is first evident here. 
That the chromosomes are really tetrads at this stage is shown still more 
conclusively in Calycanthus and Richardia. As the parts of the bivalent 
chromosomes pass toward the poles of the spindle, the two halves of the 
split chromosome become plainly visible. That the parts of each bivalent 
chromosome separate and pass to the poles as double V’s, as described by 
Flemming (’ 87 ), has long been held to be characteristic of the heterotypic 
division. These portions, according to Flemming (’ 87 ) and Meves (’ 97 ), 
undergo a longitudinal splitting as they diverge towards the poles, which 
has also been held to be characteristic of heterotypic mitoses. In Thalic - 
trum the form of each univalent chromosome as it passes toward the pole 
is that of a V, each arm of which is formed by a longitudinal splitting 
of a single rod (Fig. 27 b } PL I). All four ends of the separating chromo- 
somes are usually turned outward, away from the long axis of the spindle 
(Figs. 25, 26, and 27 a, PL I). Other figures may arise by a failure of the 
halves of each univalent chromosome to separate, or by their partial 
separation (Fig. 2 J c, Pl. I). No matter how widely separated the two 
halves of the univalent chromosomes may become in the metaphases, they 
usually eventually come to lie close together and pass to the poles as 
double rods, being more or less bent or curved (Figs. 28 and 29, PL I). 
I have made a detailed study of the later division stages of Thalictrum 
purpurascens. Fig. 28, PL I, shows the form and position of the chromo- 
somes as they approach the poles of the spindle. Each chromosome, as in 
the earlier anaphases, may be seen to be composed of two parts. Fig. 29, 
Pl. I, shows these two parts very distinctly. As the chromosomes approach 
the poles they do not crowd together, as is so often the case in many 
plants, but remain rather widely separated. They may even be counted 
with ease in many cases. This wide separation of the chromosomes also 
obtains when they come to lie in the newly-forming cavity of the daughter 
