348 
LESTER W. SHARP 
earlier stages also, the chromosomes do not form a continuous spirem. 
On the other hand, it seems probable that some of the chromosomes may- 
hang together end to end, since the number of ends which can be distin- 
guished appears to be much smaller than the number which would be 
expected if all the chromosomes were free from one another. 
The spindle now begins to differentiate in the cytoplasm, and the nuclear 
membrane contracts about the chromosomes. In the chromosomes the 
split now becomes complete, but as they continue to shorten and thicken 
their halves become so tightly pressed together that in many preparations 
they can scarcely be distinguished. As the contraction reaches its climax 
the nuclear membrane disappears (fig. 24), and the chromosomes, after 
loosening up as an irregular group, rapidly become arranged on the spindle 
with their halves clearly evident (fig. i). 
Discussion 
A number of the features of somatic mitosis as the writer has found 
them have been compared with the results of other investigators in the 
paper on Vicia (191 3) and need not be reconsidered here. In the present 
discussion attention will be limited to three important points: the time of 
chromosome splitting, the method of splitting, and the bearing of the results 
of this study on certain interpretations of the heterotypic prophase. 
Time of chromosome splitting. Because of the exact manner in which 
the telophasic and prophasic changes are seen to occur when closely exam- 
ined, the writer has contended that the definitive splitting of the chromo- 
somes occurs in the prophase rather than in the telophase as several workers 
have urged. In the first place, the telophasic a'lveolation, as emphasized 
in the foregoing description, is a very irregular process, its result being the 
transformation of each chromosome into an irregular alveolar-reticulate 
structure showing nothing which can with any justice be called doubleness. 
After an inspection of the figures of longitudinal and cross sections of the 
telophase chromosomes (figs. 5-11) further argument on this point would 
seem to be superfluous. Secondly, the alveolar-reticulate bands into which 
the resting reticulum breaks down in early prophase, and which are probably 
in all cases continuous with the similar bands (chromosomes) of the preced- 
ing telophase, are not transformed directly into the split spirems of the 
later prophase, but give rise to single threads in which the definitive split 
is formed as the result of a process which appears to begin with the develop- 
ment of an axial series of new vacuoles. It has been shown that as these 
single threads are evolved most of the vacuoles and open spaces which had 
their origin in the preceding telophase, and which constitute the openings 
in the resting reticulum, become lost through the breaking down of their 
boundaries, so that the telophasic vacuoles, whether so situated as to make 
the chromosome double or not, take little or no part in the development of 
the definitive split. 
