andchildren appear in accord with the way in
which the X chromosomes are transmitted from one generation to the next.

The _smaller classes_ of grandchildren, the exchange combinations or
cross-overs, as we call them, can be explained by the assumption that at
some stage in their history an interchange of parts has taken place between
the chromosomes. This is indicated in the diagrams.

The most important fact brought out by the experiment is that the factors
that went in together tend to stick together. It makes no difference in
what combination the members of the two pairs of characters enter, they
tend to remain in that combination.

If one admits that the sex chromosomes carry these factors for the
sex-linked characters--and the evidence is certainly very strong in favor
of this view--it follows necessarily from these facts that at some time in
their history there has been an interchange between the two sex chromosomes
in the female.

There are several stages in the conjugation of the chromosomes at which
such an interchange between the members of a pair might occur. There is
further a small amount of direct evidence, unfortunately very meagre at
present, showing that an interchange does actually occur.

At the ripening period of the germ cell the members of each pair of
chromosomes come together (fig. 49, e). In several forms they have been
described as meeting at one end and then progressively coming to lie side
by side as shown in fig. 63, e, f, g, h, i. At the end of the process they
appear to have completely united along their length (fig. 63, j, k, l). It
is always a maternal and a paternal chromosome that meet in this way and
always two of the same kind. It has been observed that as the members of a
pair come together they occasionally twist around each other (fig. 63, g,
l, and 64, and 65). In consequence a part of one chromosome comes to be now
on one side and now on the other side of its mate.

[Illustration: FIG. 63. Conjugation of chromosomes (side to side union) in
the spermatogenesis of Batracoseps. (After Janssens.)]

When the chromosomes separate at the next division of the germ cell the
part on one side passes to one pole, the part on the other to the opposite
pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this
time there must result an interchange of pieces where they were crossed
over each other.

[Illustration: FIG. 64. Scheme to illustrate a method of crossing over of