Fig.91 A, B, C. Vertical section of the dorsal part of three triton-embryos. Fig. 91. _A, B, C._—Vertical section of the dorsal part of three triton-embryos. (From _Hertwig._) In Fig. _A_ the medullary swellings (the parallel borders of the medullary plate) begin to rise; in Fig. _B_ they grow towards each other; in Fig. _C_ they join and form the medullary tube. _mp_ medullary plate, _mf_ medullary folds, _n_ nerve-tube, _ch_ chorda, _lh_ body-cavity, _mk_1 and _mk_2 parietal and visceral mesoblasts, _uv_ primitive-segment cavities, _ak_ ectoderm, _ik_ entoderm, _dz_ yelk-cells, _dh_ gut-cavity. From the evolutionary point of view the cœlom-pouches are, in any case, older than the chorda; since they also develop in the same way as in the chordonia in a number of invertebrates which have no chorda (for instance, _Sagitta,_ Figs. 76–78). Moreover, in the amphioxus the first outline of the chorda appears later than that of the cœlom-sacs. Hence we must, according to the biogenetic law, postulate a special intermediate form between the gastrula and the chordula, which we will call _ cœlomula,_ an unarticulated, worm-like body with primitive gut, primitive mouth, and a double body-cavity, but no chorda. This embryonic form, the bilateral _cœlomula_ (Fig. 81), may in turn be regarded as the ontogenetic reproduction (maintained by heredity) of an ancient ancestral form of the cœlomaria, the _ Cœlomæa_ (cf. Chapter XX). In _Sagitta_ and other worm-like animals the two cœlom-pouches (presumably gonads or sex-glands) are separated by a complete median partition, the dorsal and ventral mesentery (Fig. 78 _dm, vm_); but in the vertebrates only the upper part of this vertical partition is maintained, and forms the dorsal mesentery. This mesentery afterwards takes the form of a thin membrane, which fastens the visceral tube to the chorda (or the vertebral column). At the under side of the visceral tube the cœlom-sacs blend together, their inner or median walls breaking down and disappearing. The body-cavity then forms a single simple hollow, in which the gut is quite free, or only attached to the dorsal wall by means of the mesentery.