The embryology of amphioxus

AbstractAlthough the egg of Amphioxus is much more fluid and less stereotyped than that of Ascidians, the poles, axes, and localizations of formative materials are much the same in the two. The spermatozoon enters near the vegetative pole and a peripheral layer of granular cytoplasm flows to this pole and later forms a crescent around the posterior side parallel to the first cleavage amphiaster; this is the mesodermal crescent. On the anterior side a similar area later gives rise to the chorda‐neural crescent. Above these crescents is the ectodermal area, below them the endodermal area. The early cleavages divide these crescents and areas just as in Ascidians. The coeloblastula is at first spherical, but later flattens in the region of the mesodermal crescent; this flattening extends forward on the vegetative side to the chorda‐neural crescent, where the invagination is sharpest. The blastopore is at first triangular in outline, the dorsal lip being formed by the chorda‐neural crescent and the lateral lips by the mesodermal crescent. Later the chorda and the mesodermal crescents are infolded, the lateral lips fuse to form the ventral lip which grows dorsalward, the blastopore becomes wider from right to left than dorsoventrally, the gastrula elongates and in the angles between dorsal and ventral lips the mesodermal crescent forms the mesodermal grooves in the lateral walls of the gastrocoel.