I give the name of _chordula_ or _chorda-larva_ to the embryonic stage of the vertebrate organism which is represented by the amphioxus larva at this period (Figs. 83, 84, in the third period of development according to Hatschek). (Strabo and Plinius give the name of _cordula_ or _cordyla_ to young fish larvæ.) I ascribe the utmost phylogenetic significance to it, as it is found in all the chorda-animals (tunicates as well as vertebrates) in essentially the same form. Although the accumulation of food-yelk greatly modifies the form of the chordula in the higher vertebrates, it remains the same in its main features throughout. In all cases the nerve-tube (_m_) lies on the dorsal side of the bilateral, worm-like body, the gut-tube (_d_) on the ventral side, the chorda (_ch_) between the two, on the long axis, and the cœlom pouches (_c_) at each side. In every case these primitive organs develop in the same way from the germinal layers, and the same organs always arise from them in the mature chorda-animal. Hence we may conclude, according to the laws of the theory of descent, that all these chordonia or chordata (tunicates and vertebrates) descend from an ancient common ancestral form, which we may call _Chordæa._ We should regard this long-extinct _Chordæa,_ if it were still in existence, as a special class of unarticulated worm (_chordaria_). It is especially noteworthy that neither the dorsal nerve-tube nor the ventral gut-tube, nor even the chorda that lies between them, shows any trace of articulation or segmentation; even the two cœlom-sacs are not segmented at first (though in the amphioxus they quickly divide into a series of parts by transverse folding). These ontogenetic facts are of the greatest importance for the purpose of learning those ancestral forms of the vertebrates which we have to seek in the group of the unarticulated vermalia. The cœlom-pouches were originally sexual glands in these ancient chordonia. Figs. 87 and 88. Diagrammatic vertical section of coelomula-embryos of vertebrates. Figs. 87 and 88—Diagrammatic vertical section of cœlomula-embryos of vertebrates. (From _Hertwig._) Fig. 87, vertical section _through_ the primitive mouth, Fig. 88, vertical section _before_ the primitive mouth. _u_ primitive mouth, _ud_ primitive gut. _d_ yelk, _dk_ yelk-nuclei, _dh_ gut-cavity, _lh_ body-cavity, _mp_ medullary plate, _ch_ chorda plate, _ak_ and _ik_ outer and inner germinal layers, _pb_ parietal and _vb_ visceral mesoblast. Figs. 89 and 90. Transverse section of coelomula embryos of triton. Figs. 89 and 90—Transverse section of cœlomula embryos of triton. (From _Hertwig._) Fig. 89, section _through_ the primitive mouth. Fig. 90, section in front of the primitive mouth, _u_ primitive mouth. _dh_ gut-cavity, _dz_ yelk-cells, _dp_ yelk-stopper, _ak_ outer and _ik_ inner germinal layer, _pb_ parietal and _vb_ visceral middle layer, _m_ medullary plate, _ch_ chorda.