Katharine Foot and E. C. Strobell 289 
sults in this particular supporting Wilson, who also finds this chromo- 
some showing exceptionally the tetrad form. It is difficult to conceive 
why this chromosome should appear even exceptionally as a tetrad, if 
it is destined not to divide in the second spindle, for if the tetrad form of 
these stages indicates a double division of the chromosome, and even if 
such a character appears only exceptionally it seems to us it should have 
the weight of positive as against negative evidence. 
In Photo. 5 the eccentric chromosome is on the left periphery of the 
group and although its characteristic form is somewhat obscured its 
identity is beyond question. In Photo. 6 it is on the left periphery of 
the group—just above a chromosome which has separated transversely. 
In this preparation the typical double-rod form of the eccentric chromo- 
some is clearly defined. In Photo. 7 it is on the right periphery of the 
group—just below and to the right of the upper microchromosome. In 
Photo. 8 it is on the left periphery of the group—just below and to the 
right of the upper microchromosome. In Photo. 9 it is on the upper 
periphery of the group—its typical double-rod form making its identi- 
fication unquestionable. Just above and to the right of the eccentric is 
a denser chromosome showing a distinct transverse furrow, and in the 
following stages such a dense dyad can be identified in many of the 
preparations and it is clearly differentiated from the eccentric chromo- 
some. In later stages, however (prophases of second spindle), it is pos- 
sible to confuse such a dyad with the eccentric chromosome, and for this 
reason we would call attention to it in all these earlier stages in which 
such a confusion of the two is impossible. Occasionally, more than one 
of the ten large chromosomes has the form of a dense dyad, but the point 
we wish to make is that it is not the eccentric chromosome which as- 
sumes this form, and we shall, therefore, call attention to those prepara- 
tions in which the two forms are present and clearly differentiated from 
each other. This is the case in Photo. 10, in which the eccentric chromo- 
some shows its typical form of two thin parallel rods, the furrow between 
these rods forecasting the plane of its division in the first spindle. To 
the left of the eccentric is a dense chromosome with an indication of a 
transverse constriction and above is another chromosome contracted into 
almost a dyad form, yet neither of these chromosomes can possibly be 
confused with the eccentric. In Photo. 11 the eccentric has its position 
typical of the late prophase or metaphase stages, 7. ¢., outside the circle 
of nine chromosomes. In common with the other chromosomes at this 
stage, it has contracted into a smaller denser body, but the line of its 
21 
