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-symmetrical met with in Vertebrates. The nervous system Semper found to develop in the buds of _Nais_ and _Chaetogaster_ by an ectodermal thickening, just as in some Vertebrates. The cerebral ganglion was formed by the ends of the nerve-cord growing up round the oesophagus and fusing with the paired "sense-plates" which develop from the ectoderm of the head. The cerebral ganglion is accordingly only secondarily haemal in position, and there is no need therefore to seek in Vertebrates for the homologue of the oesophageal commissures of Annelids, as, for instance, Schneider did.