ition to the three
great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper
follows Geoffroy's lead very closely in maintaining that it is not the
position of the organs relative to the ground that must be taken into
account in establishing their homologies, but solely their spatial
relations one to another. He holds that dorsum and venter are terms of
purely physiological import, and he proposes to substitute for them the
terms neural and cardial (better, haemal) surfaces, either of which may
be either dorsal or ventral in position.

Having established this primary principle, Semper has little difficulty
in showing that the main organs of the body lie to one another in the
same relative positions in Annelida, Arthropoda, and Vertebrata; and
this, together with the metameric segmentation common to them all,
constitutes his first great argument in favour of their genetic
relationship. But he has still to show that Annelids possess at least
the rudiments of certain organs which seem to be peculiar to
Vertebrates, as the gill-slits, the notochord, and a nervous system
developed from the ectoderm of the "dorsal" surface. He takes particular
cognisance also of the old distinction drawn by von Baer, that
Vertebrates show a "double-symmetrical" mode of development (_evolutio
bigemina_), the dorsal muscle-plates forming a tube above the notochord,
the ventral plates a tube below the notochord, whereas Articulates do
not possess this axis, and form only one tube, namely, that round the
"vegetative" organs (_evolutio gemina_). Semper is at pains to prove
that _evolutio bigemina_ is characteristic also of Annelidan
development.

[Illustration: FIG. 14.--Transverse Section (Inverted) of the Worm
_Nais_. (After Semper.)]

He gets his facts from an elaborate study of the process of budding in
the _Naidae_, making the somewhat risky assumption that regeneration
takes essentially the same course as embryonic development.

He succeeds in showing--to his own satisfaction at least--that in the
formation of new segments in _Nais_ and _Chaetogaster_ a strand of cells
appears between the alimentary canal and the nerve-cord, and that from
this axial strand the haemal muscle-plates grow out dorsally round the
alimentary canal and the neural muscle-plates ventrally round the
nerve-cord (see Fig. 14).

This strand of cells, he concludes, must clearly be the notochord, and
the type of development is obviously the double-symm