As soon as the cord becomes free from the epiblast, it becomes surrounded by a sheath, formed of somatic mesoblast. In each of the ganglionic enlargements there next appears on the dorsal surface a pair of areas of punctiform material, the substance of which soon differentiates itself into nerve-fibres. These areas, by uniting from side to side, give rise to the transverse commissures, and also by a linear coalescence to the longitudinal commissures of the cord. The cellular parts of the band surrounding them become converted into a ganglionic covering of the cord.
In each ganglion the cells of this ganglionic investment penetrate as a median septum into the cord. A fissure is next formed, dividing this septum into two; it is subsequently continued for the whole length of the cord.
Arthropoda. In the Tracheata and the Crustacea the development of the ventral cord is in the main similar to that in the Chætopods, while that of the supraœsophageal ganglia is as a rule somewhat more complicated. No such clear evidence of an independent development of these two parts, as in the case of the Chætopods, has as yet been produced.
The most primitive type of nervous system amongst the Tracheata is that of Peripatus, where it consists of large supraœsophageal ganglia, continuous with a pair of widely separated but large ventral cords united posteriorly above the anus. These cords have an investment of ganglion-cells for their whole length, and are imperfectly divided into ganglia corresponding in number with the feet.
Fig. 241. Section through the trunk of an embryo of Peripatus. The embryo from which the section is taken was somewhat younger than that of fig. 242.
sp.m. splanchnic mesoblast; s.m. somatic mesoblast; mc. median section of body cavity; lc. lateral section of body cavity; v.n. ventral nerve cord; me. mesenteron.
The ventral cords are formed as two separate epiblastic ridges ([fig. 241], v.n), continued in front into a pair of thickenings of the procephalic lobes, which are at first independent of each other, and from which a large part of the supraœsophageal ganglia takes its origin. After the latter have become separated from the epiblast an invagination of the epiblast covering them grows into each lobe ([fig. 242]), and becoming constricted from the superficial epiblast, which remains as the epidermis, forms a not unimportant part of the permanent supraœsophageal ganglia.
In the Arachnida the mode of development of the nervous system is essentially the same, and the reader will find a detailed account of it for Spiders in Vol. II. pp. 447-451. The ventral cords are here formed as independent and at first widely separated strands ([fig. 243], vn), which for a long time remain far apart; they are subsequently divided into ganglia and become united by transverse commissures.
The supraœsophageal ganglia are formed as two independent thickenings of the procephalic lobes ([fig. 244]), which eventually separate from the superficial skin. There is formed however in each of them a semicircular groove ([fig. 244], gr) lined by the superficial epiblast, which becomes detached from the skin, and is involuted to form part of the ganglia.