138 
Digby. — On the Archesporial and 
which constitute nuclear mitosis. This double character is apparent more 
or less continuously, except during the ‘ rest ’ which may intervene between 
successive nuclear divisions, when it may become temporarily obscured. It 
is represented by paired threads, by paired beads, and by paired segments 
of spireme. In order that this duality may be clearly appreciated, it is pro- 
posed to trace its course briefly before commencing the detailed description 
of a somatic division (see Text-fig., Nos. 1-8). 
If a nuclear division be examined at late anaphase or telophase it will 
be seen that the individual daughter chromosomes, which have recently 
separated on the equatorial plate, show fission in varying degrees, thus 
tending to divide their substance into longitudinal halves (threads). As the 
newly-formed nuclei prepare to pass into rest the sides or halves (threads) of 
the chromosomes become beaded, with lengths of linin between the beads. 
The sides gradually separate from one another and the beads become 
resolved into ever finer granules, until in complete rest a state of more or 
less even reticulum is reached. In some plants there is no such thorough 
chromosome dissolution, and portions of split chromosomes may persist and 
thus establish visible continuity between anaphase and prophase. During 
prophase the series of events is reversed, and the chromosome halves 
(threads) which separated during the preceding telophase approach one 
another, forming parallel groupings of granules and parallel threads. The 
space between each pair of associating parallels becomes the future line of 
fission. When association is consummated in the realization of the com- 
pleted chromosome, each chromosome opens out and splits into longitu- 
dinal halves on the equatorial plate of the spindle, and each half becomes 
a daughter chromosome. The daughter chromosomes proceed to the poles, 
and thus the cycle is completed. 
Such is the scheme of a typical somatic mitosis, and it will be shown 
later how this simple procedure is elaborated in the heterotype division. 
Occasionally, as in Primula ( 4 ), a modification occurs, owing to the longi- 
tudinal halves (threads) of the chromosomes tending to hold together, 
instead of to separate. Accordingly, the entire chromosome in telophase 
inclines to break up transversely into beads or segments, instead of to split 
into two parallel threads. When the nucleus enters upon prophase, the 
chromosomes are reorganized by the entire univalent segments co'ming 
together again end to end like a string of beads. 
The archesporium has been selected for the study of vegetative divi- 
sions, and the description of the cycle of phases passed through by a nucleus 
will commence with the anaphase. This is a clearly defined stage with the 
daughter chromosomes newly arrived at the poles, and thus affords a good 
starting-point whence to trace the dissolution and subsequent reconstruc- 
tion of the chromosomes. 
The daughter chromosomes, having arrived at the spindle poles, at first 
