Gates. — Somatic Mitoses in Oenothera . 999 
cells occur only occasionally, and no explanation of their occurrence is to 
be offered in the present paper. 
Metaphase. 
Fig. 16 represents a normal metaphase in slightly oblique view. The 
chromosomes are not all represented, but the longitudinal split can be clearly 
seen. Figs. 17-21 represent equatorial plates of the metaphase in polar 
view. Figs. 17 and 18 each contain 15 chromosomes, while 16 chromosomes 
are present in Fig. 19, 12 in Fig. 20, and 20 or 21 in Fig. 21, though for the 
sake of clearness only 18 are drawn in the figure. As already stated, the 
regular number in this plant is undoubtedly 15, and the departures shown 
in Figs. 19, 20, and 21 represent rare cases. Such wide fluctuations as 
12 and 20 were particularly surprising to find, but accurate study of other 
somatic tissues will probably disclose equally striking cases of occasional 
variations in number in certain stages of mitosis. 
Farmer and Shove (’ 05 ) described similar variations in the root tips 
of Trade sc antia virginica . After a large series of counts they found the 
chromosome number to vary from 26 to 33 (p. 56 2). They also (p. 563) 
found the number inconstant during the heterotype mitosis, varying from 
12 to 16. This may be connected with the fact that the plant commonly 
fails to set seed. 
Wilson (’ 09 , p. 185), in his study of the chromosomes in the 
Hemipteran genus Metapodius, also found occasional variations of one in 
the number of chromosomes in the spermatogonia and ovaries. 
It will be seen that in Fig. 20 two of the chromosomes, and in Fig. 21 
one chromosome, show a precocious split. As a possible suggestion 
regarding the origin of the increased number of chromosomes in Figs. 19 
and 2r, we may assume that the fission which regularly occurs in the 
prophase chromosomes in diakinesis, instead of closing up, becomes accentu- 
ated, resulting in the separation of those daughter chromosomes and their 
regular orientation in a single plane afterwards, on the equatorial plate of 
the spindle. Of course it does not follow that such chromosomes will 
undergo another fission on the spindle, and thus perpetuate the increased 
number. It might well be, and is indeed perhaps more probable, that those 
chromosomes which, on our hypothesis, completed their precocious fission 
and separation in the prophase, would merely travel to opposite poles of the 
spindle and thus restore the normal number. The determination of this 
possibility by observations of fixed material is practically an impossibility, 
for the moment such chromosomes pass towards the poles in anaphase, there 
remains no evidence that they separated immediately after the precocious 
split in the prophase. The probability of this explanation is, however, 
increased by the fact that the aberrant numbers were, all but one, found in 
metaphase groups. 
