1. The secondary spermatogonia are much reduced in size at the end of
their divisions and the cytoplasm is very small in amount. The
rod-shaped chromosomes number thirty-three, and, of these, one is to be
distinguished from its fellows by greater size and slower division.
2. From the substance of the disintegrated spermatogonial chromosomes,
the tetrads of the first spermatocytes are formed. It was impossible to
determine the relation of the elements of the two generations, but the
changes are rapid and there is no intervening resting condition of the
nucleus.
3. It could not be determined whether or not the spireme is continuous.
A longitudinal split appears very early, and shortly after the chromatin
segments may be seen. These soon betray at their centers an indication
of the cross-division, producing crosses with arms that may vary
considerably in relative lengths. No reason was found for considering
both divisions longitudinal.
4. The typical element is granular and more or less rod-shaped, with the
longitudinal division merely indicated by a narrow line, and with but
slight elongation of the chromatids along the plane of the
cross-division. Various modifications of this occur, by which the
longitudinal cleft is much increased in width at the center, the
cross-arms are greatly extended, or approximation of the ends of the rod
brought about, producing a ring.
5. The definitive chromosomes of the metaphase are produced by a
concentration of the prophase elements, whereby they become shorter,
heavier, and entirely homogeneous in structure. Distinct lines of
division between the chromatids are not visible, but the tetrad
character of the elements is readily established by observing the steps
in their formation.
6. The accessory chromosome early becomes distinguishable because of its
peripheral position and strong tendency to stain with safranin, while
the remaining chromatin takes the gentian violet by Flemming’s
three-color method. At first it appears as a homogeneous plate, but
later this is seen to be a closely coiled thread. As the chromatin
segments shorten and broaden to form the chromosomes of the mitotic
figure, this thread also grows shorter and heavier until it forms an
element of essentially the same character as that of the spermatogonial
chromosome from which it was derived.
7. Upon the establishment of the mitotic figure, the chromosomes arrange
themselves in the equatorial plate with their longer axis perpendicular
to the spindle axis. Division of the elements is not synchronous, so
that all stages of the chromatid movements may be observed in one
nucleus. By this means it is possible to determine that separation of
the chromosomes takes place along the plane which marked the
longitudinal division of the prophase thread in such a way that the
chromatids show no clear interspaces. The individual chromosome near the
end of its division has the same form as that with which it started,
except for the difference that the chromatids are now in contact for the
greater part of their length along the plane of their cross-division. As
the daughter chromosomes separate, this line of division comes into
evidence through the springing apart of the two chromatids now composing
each chromosome. The result is the formation of two V-shaped chromosomes
with mantle fibers attached to their apices. The accessory chromosome
does not participate in this division, but passes unchanged to one pole
of the spindle.
8. By reason of the action of the accessory chromosome in the first
spermatocyte mitosis, there are produced two numerically equal classes
of second spermatocytes—(_a_) those containing sixteen dyad chromosomes
and an undivided accessory chromosome, and (_b_) those with merely the
sixteen dyad elements. In both cases the mitotic figure quickly reforms
without an intervening rest stage in which the chromosomes lose their
identity. There is a loosening up of the chromomeres in all the elements
except the accessory chromosome, so that they have a structure and
staining reaction similar to that of the first spermatocyte chromosomes
just before they enter the metaphase. The dyads of the first
spermatocyte telophase, and of the succeeding and greatly abbreviated
second spermatocyte prophase, are quite as definite structures as are
the chromosomes of the first spermatocyte prophase.
9. All the chromosomes of the second spermatocyte are paired structures
and divide in a similar way. The spindle is small and weak as compared
with that of the first spermatocyte, and the chromosomes arrange
themselves radially on its periphery in such a way that the pairs lie in
the plane of the spindle axis with their joined ends inward. The space
between the chromatids represents the line of cross-division observable
in the prophase segments of the first spermatocyte, and their separation
accordingly represents a reduction division. The accessory chromosome,
on the contrary, divides along the plane marking the longitudinal cleft
of the spermatogonial spireme.
10. From each first spermatocyte there are formed, by two divisions,
four spermatids, of which two are distinguished from the remaining pair
by the possession of an extra chromosome in addition to the
number—sixteen—common to them all. Both classes undergo a like series of
transformations by which they become mature spermatozoa. These are
necessarily of two kinds; and it is believed that those containing the
accessory chromosome, in the act of fertilizing the egg, determine that
the germ-cells of the embryo shall be sexually male, or like themselves,
while those from which it is absent are unable to impress their sex upon
the egg and assist in producing female embryos.