The hypoblastic walls of the archenteron give off a long median dorsal groove which becomes the notochord (Fig. 84, C and D, _ch_); and also an anterior pouch and certain lateral pairs {132}of diverticula which are the enterocoeles or coelomic pouches, and give rise to the mesoblastic somites (Fig. 84, B and C, _mk_). The notochord (Fig. 84, D, _ch_) is at first a longitudinal cellular ridge, which becomes segmented off from the hypoblast as a rod lying below the neural canal. It is seen in various stages of development in Figs. 84 and 86, leading to the vacuolated condition of the adult. [Illustration: FIG. 84.—Four stages in the development of the notochord, nervous system, and mesoderm of Amphioxus. _ak_, Ectoderm; _ch_, notochord; _dh_, cavity of archenteron; _hb_, ridge of ectoderm growing over medullary plate; _ik_, endoderm; _lh_, coelom; _mk_, coelomic pouch; _mk^1_, parietal layer of mesoderm; _mk^2_, visceral layer; _mp_, medullary plate; _n_, neural canal; _ns_, protovertebra. (From Korschelt and Heider, after Hatschek.)] The coelomic pouches are five in number—(1) one median, anterior, which gives rise to the two head cavities, the left-hand one of which opens to the exterior by means of the pre-oral pit; (2) a pair of small lateral pouches, placed anteriorly and dorsally, which do not divide but give rise to the first pair of myotomes only and their outgrowths which extend back into the metapleural folds, where, however, they are later replaced by lymph spaces; and {133}(3) a second pair of diverticula, more posteriorly placed, which continue to grow back towards the blastopore, and have paired mesoblastic somites, the cavities in which are the beginnings of the coelom in the body, constricted off from them successively from before backwards (Fig. 85, A, _ush_) to form all the remaining myotomes.[115] This is the first sign of segmentation in the animal, and at this stage, when it has about five pairs of mesoblastic somites, it breaks out of its covering and becomes a free-swimming larva. [Illustration: FIG. 85.—Embryo of Amphioxus. A, in vertical section, slightly to the left of the middle line. B, in horizontal section. _ak_, Ectoderm; _cn_, neurenteric canal; _dk_ and _ud_, archenteron; _ik_, endoderm; _mk_, mesodermal folds; _n_, medullary canal; _us_, first coelomic pouch; _ush_, coelomic cavity; _V_, anterior, _H_, posterior, end. (From Korschelt and Heider, after Hatschek.)] The mouth now appears, and soon grows to a large opening on the left side of the now pointed anterior end (Fig. 86, A, _m_), and the first gill-slit (_ks_) forms as a direct communication from the front of the mesenteron (pharynx) to the exterior. It is ventral at first, and then shifts over to the right side. The anus forms posteriorly, and the neurenteric canal closes {134}up. A depression on the floor of the enteron close to the mouth gives rise to the "club-shaped gland" (Fig. 86, B, _k_), which is probably a gill-cleft in its nature.