The primordium of the neurochord (neural or medullary plate) referred to above becomes closed in from the surface by the overgrowth of surrounding epiblast, and its edges also bend up, meet, and finally fuse to form a tube, the medullary or neural tube.
These are a ventral arch forming a neural canal through which the great nerve cords pass (figs.
The work of Camillo Golgi (Pavia, 1885 and onwards) on the minute structure of the nervous system has led to great alteration of doctrine in neural physi nerve cells, that is to say, the fine nerve fibres - since all nerve fibres are nerve cell branches, and all nerve cell branches are nerve fibres - which form a close felt-work in the nervous centres, there combined into a network actually continuous throughout.
Na, Neural arch; ch, chorda; pl, pleurocentrum; ic, intercentrum.
It is still longer before the neural tube completes its closure in front, exhibiting a small orifice at the surface, the anterior neuropore.
N, Neural canal; ch, chorda.
An important fact to note is that the blastopore is included in this overgrowth of epiblast, so that the neural tube remains for some time in open communication with the archenteron by means of a posterior neurenteric canal.
A prominent and practically important illustration of neural tonus is given by the skeletal muscles.
The neural laminae are broad, the spines almost obsolete, except in the seventh, and the transverse processes not largely developed.
The vertebral axis shows a series of broad rings, with distinct neural arches, but no ribs.