gmentation spindle of the egg.

GENERAL DISCUSSION.

THE "ACCESSORY CHROMOSOME."

The literature bearing on the "accessory chromosome" of McClung, the
"small chromosomes" of Paulmier, and the "chromatin nucleoli" of
Montgomery has been fully discussed by McClung in the paper entitled,
"The accessory chromosome--sex determinant?" ('02), and will therefore
be considered here only in its relation to the several forms studied.
The present status of the question has been well summarized more
recently by Montgomery under the heading "Heterochromosomes" in the
paper, "Some observations and considerations upon the maturation
phenomena of the germ cells."

Three theories as to the function of the "heterochromosomes" have been
advanced: (1) That of McClung that they are sex-determinants, since in
the forms which he has examined these chromatin bodies occur in only
one-half of the spermatozoa, and the sex-character is the only character
which divides the individuals of a species into two approximately equal
groups. (2) That of Paulmier and Montgomery that they are degenerating
chromatin. Montgomery regards them as "chromosomes that are in the
process of disappearance in the evolution of a higher to a lower
chromosome number." (3) That of Miss Wallace, who suggests that in the
spider only the one out of each four spermatids which contains the
accessory chromosome is capable of developing into a functional
spermatozoon, while the other three degenerate, as do the polar bodies
given off by the egg. McClung is inclined to believe that the accessory
chromosome is an element common to all of the male reproductive cells of
Arthropods, and probably to vertebrate spermatocytes as well ('02).

Of the insects considered in this paper _Aphis_ and _Termopsis_ have no
"accessory chromosome" or "heterochromosome" of any kind. The fact that
no males develop from the fertilized eggs of _Aphis_ might be offered as
a reason for its absence, but such an argument would not apply to
_Termopsis_. The sex-character may indeed be represented in the
chromatin of some one of the pairs of paternal and maternal chromosomes
of the spermatocytes, but there is no evident peculiarity by which
one-half of the spermatozoa can be said to be different from the other
half. As to McClung's statement ('02) "that if there is a cross-division
of the chromosomes in the maturation mitosis, there must be two kinds of
spermatozoa, regardless of the presence of th