Fig. 86.—A, young larva of Amphioxus. B, anterior end enlarged. c, Provisional tail-fin; ch, notochord; cn, neurenteric canal; d, enteron; h, coelom of snout; k, club-shaped gland; k′, its external aperture; ks, first gill-slit; m, mouth; mr, nerve-tube; np, neuropore; sv, sub-intestinal vein; w, pre-oral pit. (After Hatschek.)
Fig. 87.—More advanced larva of Amphioxus. an, Anus; au, eye-spot; c, larval tail-fin; ch, notochord; d, enteron; fl, rudiment of endostyle; k, club-shaped gland; k′, its external aperture; m, mouth; np, neuropore; w, pre-oral pit; x, provisional nephridium; 1-4, gill-slits. (From Korschelt and Heider, after Lankester and Willey.)
The walls of the coelomic pouches, which have been extending both dorsally and ventrally (Fig. 84, D), become the mesoderm, the outer the somatic and the inner the splanchnic layer; and the ventral parts of their cavities unite to form the coelom. The cells of the dorsal parts become muscle fibres, and constitute the myotomes internally and the connective tissue of the skin externally.
The larva (Fig. 87) is now long and narrow with many segments, pointed ends, and a caudal fin. The gill-slits all appear first in the mid-ventral line and then shift over to the right side (Fig. 87, 1-4): they are metamerically arranged. After fourteen have been so formed a series of eight appear dorsally to those on the right side, and then the first set, originally ventral, move over to the left side, and by the suppression of some they become equal in number and segmentally arranged on the two sides of the body. This is perhaps the stage at which Amphioxus shows the nearest approach to the typical embryo of a higher Vertebrate. The gill-slits are here seven to nine on each side, and the Vertebrate embryo has usually five to seven on each side. These first gill-slits in Amphioxus are later subdivided by the downgrowth of the tongue-bar from the dorsal edge.
Fig. 88.—Ventral aspect of three larvae of Amphioxus, showing the metapleural folds and the formation of the atrium. ap, Atriopore; k, gill-slits; lf and rf, left and right metapleural folds; m, mouth; w, pre-oral pit. (From Korschelt and Heider, after Lankester and Willey.)
The atrium is an ingrowth of the external space between the two ventral metapleural or atrial folds (Figs. 88 and 89), paired lateral ridges of the body-wall, and so is lined by ectoderm. This ingrowth is shut off from the exterior by the growth towards each other of sub-atrial ridges on the inner sides of the metapleural folds (see Fig. 89, A, sl), and then becomes greatly enlarged by the increased relative growth of the ventro-lateral part of the body-wall (Fig. 89, B, C). The posterior opening between the metapleural folds remains as the atriopore (Fig. 88, C, ap); while the anterior end (Fig. 88) also remains open for some time, but eventually closes. As the metapleural folds lie outside the gill-slits (Fig. 88, A) when these folds close in (B and C), it comes about that the gill-slits which formerly opened freely to the exterior now open into the cavity of the atrium (compare Figs. 87 and 88).
