The embryology of amphioxus

Abstract

SIXTEEN PLATES (TWO HUNDRED FIGURES) AUTHOE'S ABSTMCTAlthough the egg of Amphioxus is much more fluid and less stereotyped than that of Ascidians. the poles, axes, and localizations of formative materiala are much the same in the two. The spermatozoon enters near the vegetntive 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 t o 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 coelohlastula is at first spherical. but later flattens i n 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 a t first triangular i n outline, the dorsal lip being formed by the chorda-neural crescent and the lateral lips by the mesodermal crescent. Later the chorda and 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 elongate; and i n the angles between dorsal and ventral lips the mesodermal crescent forms the mesodermal grooves in the lateral walls of the castrocoel.

Polychaete trunk neuroectoderm converges and extends by mediolateral cell intercalation

“…This transformation is driven by convergent extension movements, the simultaneous narrowing and lengthening of the dorsal tissue (4-7). Similar morphogenetic movements were described in ascidians (8) and may also take place in amphioxus (9). Aside from chordates, however, little is known about the molecular and cellular mechanisms that underlie the elongation of spherical embryos and larvae.…”

The mesenchymal cell, its role in the embryo, and the remarkable signaling mechanisms that create it

“…The protochordate Amphioxus (Conklin, 1932;Fig. 2B-F) and amphibians form a blastopore through which presumptive mesodermal and endodermal epithelia invaginate (Fig.…”

An aboral‐dorsalization hypothesis for chordate origin

“…The early embryogenesis of amphioxus up to the late neurula stage provides us with various suggestions about chordate origins (Conklin, 1932;Hatschek, 1893;Hirakow and Kajita, 1994;Whittaker, 1997) (see Fig. 3).…”

“…Amphioxus embryology. (a) Blastula; (b) initial gastrula; (c) mid gastrula; (d) mid-to-late gastrula; (e) initial neurula, sagittal section (e) and cross section (e 0 ); (f) mid neurula, longitudinal section (f) and cross section (f 0 ); (g) mid-to-late neurula; and (h) late neurula (from Hatschek, 1893; Conklin, 1932;Hirakow and Kajita, 1994). Their radially symmetrical cleavage pattern to form a hollow blastula resembles that of acorn worms (hemichordate) (a), and their chordate affinities appear at the gastrula and neurula stages (b-h).…”