Electron microscopic study of the development of amphioxus, Branchiostoma belcheri tsingtauense: the neurula and larva.

The later development of Asian amphioxus was investigated by scanning and transmission electron microscopy (SEM and TEM). The age ranged from 10.5 hr (the early neurula) to 48 hr after fertilization (the larva with two gill-slits). This period was divided into 6 stages; the neurula (N) 1, 2, 3, and the larva (L) 1, 2, 3. At stage N1, the lateral edge of the flattened neural plate became stratified and the superficial layer separated from the deeper one to spread over the neural plate toward the median line where they fuse randomly with each other. The notochord and the mesoderm were formed by folding from the dorsomedian and the dorsolateral wall of the archenteron during stages N1 to N2. At stage N3, a typical triploblastic embryo was formed, consisting of the definitive ectoderm, neural tube, notochord, endoderm and mesoderm (the wall of the coelom). Ultrastructurally, paramyosin fibrils in the notochordal cells and myofibrils in the muscle cells were found at stage L1. After stage L1, the anteroposterior differentiation took place and the formation of the larval organs progressed. Self-feeding larval life was established at stage L3 when the larva acquired the mouth and two gill-slits.