. Meanwhile out of the
remainder, the first six pairs severally unite in the median line, while
the rest remain more or less separate. Of these six double ganglia thus
formed, the anterior four coalesce into one mass; the remaining two
coalesce into another mass; and then these two masses coalesce into one.
Here we see longitudinal and transverse integration going on
simultaneously; and in the highest crustaceans they are both carried
still further. The _Vertebrata_ exhibit this transverse integration in
the development of the generative system. The lowest of the
mammalia--the _Monotremata_--in common with birds, have oviducts which
towards their lower extremities are dilated into cavities severally
performing in an imperfect way the function of a uterus. "In the
_Marsupialia_, there is a closer approximation of the two lateral sets
of organs on the median line; for the oviducts converge towards one
another and meet (without coalescing) on the median line; so that their
uterine dilatations are in contact with each other, forming a true
'double uterus.' ... As we ascend the series of 'placental' mammals, we
find the lateral coalescence becoming gradually more and more
complete.... In many of the _Rodentia_, the uterus still remains
completely divided into two lateral halves; whilst in others, these
coalesce at their lower portion, forming a rudiment of the true 'body'
of the uterus in the Human subject. This part increases at the expense
of the lateral 'cornua' in the higher Herbivora and Carnivora; but even
in the lower Quadrumana, the uterus is somewhat cleft at its
summit."[6] And this process of transverse integration, which is still
more striking when observed in its details, is accompanied by parallel
though less important changes in the opposite sex. Once more; in the
increasing commissural connexion of the cerebral hemispheres, which,
though separate in the lower vertebrata, become gradually more united in
the higher, we have another instance. And further ones of a different
order, but of like general implication, are supplied by the vascular
system.

Now it seems to us that the various kinds of integration here
exemplified, which are commonly set down as so many independent
phenomena, ought to be generalized, and included in the formula
describing the process of development. The fact that in an adult crab,
many pairs of ganglia originally separate have become fused into a
single mass, is a fact only second in s