Fig. 5.

The development of the occipitospinal neural crest, based on photomicrographs of serial sections. (A) The spinal crest at stage 12 does not extend beyond the dermatomyotomes. The main (ventrolateral) migratory pathway detectable from sections is shown by an arrow. (B) The crest at stage 13 has given rise to spinal ganglia. The myotomic plates are found laterally, and ventrally the sclerotomes show dense and loose portions. Motor fibres penetrate the loose portions. (C) The neural tube at stage 15 is covered by pia mater, believed to be derived from the neural crest (Sensenig, 1951; O’Rahilly and Müller, 1986). Dorsal and ventral roots are forming a spinal nerve. (D) Lateral view of the occipitospinal transition at stage 15. Spinal ganglia and sclerotomes are in register. No ganglia form in the occipital part. Nerve fibres of XII pass through the loose moieties of the sclerotomes, as do also the spinal nerves. Occipital and spinal sclerotomes are in register. (The dense areas will develop into the lateral parts of the basioccipital and into the neural arches of the cervical vertebrae.)