It is only in the case of members of the former set of groups that any adequate observations have yet been made on the development of the nervous system, and even in the case of these groups observations which have any claim to completeness are confined to certain members of the Chætopoda, the Arthropoda and the Mollusca. An account of imperfect observations on other forms, where such have been made, will be found in the systematic part of this work.
Chætopoda. We are indebted to Kleinenberg (No. [329]) for the most detailed account which we have of the development of the central nervous system in the Chætopoda.
Fig. 239. Section through the head of a young embryo of Lumbricus trapezoides. (After Kleinenberg.)
c.g. cephalic ganglion; cc. cephalic portion of the body cavity; x. œsophagus.
The supraœsophageal ganglion with the œsophageal commissure developes independently of the ventral cord. It arises as an unpaired thickening of the epiblast, close to the dorsal side of the œsophagus at the front end of the head ([fig. 239]), which becomes separated from the epiblast, and extends obliquely backwards and downwards in a somewhat arched form; its lower extremities being somewhat swollen. The inner portion of this curved rudiment becomes converted into commissural nerve-fibres, while the cells of the outer and upper portion assume the characters of ganglion-cells. The commissural fibres are continued downwards to meet the ventral chord, but their junction with the latter structure is not effected till late in embryonic life.
The ventral cord is formed by the coalescence of a pair of linear cords, the development of which takes place from before backwards, so that when their anterior part is well developed their posterior part is hardly differentiated. These cords arise, one on each side of a ventral ciliated furrow, first as a single row of epiblast cells, and subsequently as several rows ([fig. 240], Vg). While still united to the external epiblast, they extend themselves below the cells lining the ventral furrow, and unite into a single nervous band, which however exhibits its double origin by its bilobed section. Before the two cords unite, the groove between them becomes somewhat deep, but subsequently shallows out and disappears. The nervous band, before separating from the epiblast, exhibits, in correspondence with the mesoblastic segments, alternate swellings and constrictions. The former become the ganglia, and the latter the connecting trunks.
Fig. 240. Section through part of the ventral wall of the trunk of an embryo of Lumbricus trapezoides. (After Kleinenberg.)
m. longitudinal muscles; so. somatic mesoblast; sp. splanchnic mesoblast; hy. hypoblast; Vg. ventral nerve-cord; vv. ventral vessel.