Soon after the appearance of the primitive strips, the first traces of segmentation may be detected. The ventral cord is from the first in direct connection and continuous with the brain. From the segmental expansions of the primitive strip arise the ventral nervous ganglia, and from the intersegmental constrictions are developed the paired longitudinal commissures.
Transverse sections of the ectoderm in the region of the primitive strips (Figs. 539, C, and 517) show several layers of cells. Of these cellular layers the deeper ones afterwards, by a kind of delamination, separate from the superficial ones and form the “lateral cords,” i.e. the germs of the longitudinal cords of the ventral ganglionic cord. Meanwhile the primitive groove (pr) deepens and forms an invagination extending between the lateral cords. The cells at the bottom of this invagination form the so-called “median cord,” and give rise to the transverse commissures connecting the ganglia.
Fig. 541.—Transverse section through the rudiment of the ventral nervous cord of Xiphidium: f, fibrous mass; m, neuroblast cells of the median cord; n1-n4, neuroblasts of the lateral cord; z, pillar of ganglion-cells arising from the neuroblasts.—After Wheeler.
Wheeler has detected in the rudiment of the ventral cord of several Orthoptera, on the upper surface of the lateral cords, four large cells which he calls neuroblasts (Figure 541, n1-n4), from which cells arise by budding and become arranged in vertically arranged layers or pillars (z). Graber has observed them in Stenobothrus and Viallanes in Mantis. These neuroblasts are only present in the inter-ganglionic region, and soon move back to the hinder side of the transverse commissures.
At first there is a pair of ganglia to each of the 16 trunk-segments of the embryo, but afterwards these become more or less fused together; thus those of the three gnathal segments unite to form the subœsophageal ganglion of the adult, and the last abdominal ganglia are fused together and move a little anteriorly (see also pp. 227, 228).
Development of the brain.—The supraœsophageal ganglion is due to the spreading out of the procephalic lobes. The rudiment of the brain is due to a thickening of the ectoderm on the sides of the mouth and of the forehead, this expansion of germinal brain-cells being the direct continuation of the primitive rolls or strips, and which finally becomes differentiated into the protocerebrum, deutocerebrum, and tritocerebrum, as stated on p. 228.
The ganglion opticum, now regarded as a part of the compound eye, arises as an ectodermal thickening on each side of the rudimentary brain. The optic ganglion belongs exclusively to the foremost division of the brain (see also p. 227).
Development of the eyes.—Compound eyes do not appear until the beginning of pupal life, the single eye (ocellus) being the primitive organ of vision. The ocellus of Acilius, according to Patten, arises as a pit or depression of the ectoderm (Fig. 542). The long hypodermal cells which form the walls of this pit or hollow are arranged in a single layer, and bear at their free ends a striated cuticular edge (c), while from their inner or basal end arise the fibres destined to form the common optic nerve.
At a later stage (Fig. 542, B), the eye-pit is closed over, the edges growing over and covering the deeper part of the eye. In this way there arises out of the pit-like rudiment a two-layered optic cup. The outer or superficial layer (l) becomes in its central part the crystalline lens, while the peripheral parts form the iris. From the cuticular striated border of these cells arise the chitinous or corneal lens. On its outer edge the superficial layer of the eye passes gradually into the unmodified hypodermis (h).