Ontogeny of the Head and Nervous System of Myxinoids

Summary

The ontogeny of myxinoids is not well documented and there are no experimental embryological data. The present contribution summarizes the available descriptive data with emphasis on the development of head and nervous system. Myxinoids lay large and polylecithalic eggs, cleavage is partial and discoidal. The endodermal pharynx initially develops a large number of pharyngeal pouches. The anterior pouches degenerate and the posterior pouches are transformed into the definite gill slits. The buccal and nasal cavities develop from an unusually shaped stomodeum that displays large lateral recesses reminiscent of typical (endodermal) pharyngeal pouches. The neuraxis develops by invagination of a neural plate and initially contains an extensive ventricular system that is reduced to a system of narrow canals and vestigial structures in adults. A well-developed dorsal neural crest and ectodermal placodes contribute to the formation of the ganglia of the cranial nerves; an additional ventral neural crest that gives rise to the optic vesicle has been observed in the rostral neural tube. A lens placode is present in the ectoderm overlying the optic cup, but disappears later without giving rise to a lens. The dorsal and ventral roots of the spinal nerves fuse in an unusual way to form mixed spinal nerves. Chorioid plexuses never develop, and the presence of a pineal anlage is doubtful. The neural crest probably contributes to the formation of a very dense pre- and postotic head mesenchyme which gives rise to cartilages and muscles, there are no anlagen of external eye muscles.