{57}[Illustration: FIG. 25.—Embryology of Ascidian. A, mature ovum: _foll_, follicle-cell; _m_, membrane; _n_, nucleus; _p_, protoplasm; _t.c_, test-cell; B, mature spermatozoon; C, segmentation-stage in section to show blastocoel; D, early gastrula-stage; E, later gastrula-stage; F, later embryo showing rudiments of notochord and neural tube; G, transverse section of body of embryo showing mesoblast and formation of neural canal; H, late embryo showing body and tail, notochord, neural canal, and mesenteron; I, young larva ready to be hatched; K, transverse section of tail of larva. _ar_, Archenteron; _at_, atrial invagination; _au_, otocyst; _b.c_, blastocoel; _b.p_, blastopore; _ch_, notochord; _ep_, epiblast; _f_, tail-fin; _hy_, hypoblast; _m.b_, mesoblast; _mes_, mesenteron; _musc_, muscle-cell; _n.c_, neural canal; _ne.c_, neurenteric canal; _n.v_, neural vesicle; _oc_, ocellus. (Modified from Kowalevsky and others.)] The embryo is elongated antero-posteriorly, the dorsal surface is flattened, and the blastopore indicates its posterior end. Around the blastopore the large ectoderm cells form a medullary plate, along which a groove (the medullary groove), runs forwards, bounded at the sides by medullary folds which meet behind the blastopore. Underneath the posterior part of the medullary groove certain of the hypoblast cells from the dorsal wall of the archenteron, in the median line, form a band extending forwards (Fig. 25, E, _ch_). This band separates off from the hypoblast, which closes in beneath it, and thus gives rise to the notochord (Fig. 25, F). The more lateral and posterior cells become mesoblast, and separate off as lateral plates, which show no trace of metameric segmentation (Fig. 25, G). The remainder of the archenteron {58}becomes the branchial sac, and by further growth buds off the rest of the alimentary canal.