Fig. 112.—Transverse section of Nerve-cord of Embryo of B. germanica (Oc. 4, Obj. D.D. Zeiss). C, cellular layer; F, fibrillar substance (punkt-substance of Leydig); Ch, cellular band; N′ N″ inner and outer neurilemma.
Nervous System.—Along the middle line of the whole ventral surface there is formed a somewhat deep groove-like infolding of the epiblast, bounded on either side by paired solid thickenings, which detach themselves from the epiblast (fig. [110], N) and constitute the double nervous chain. In many other Insects a median cord (from which are derived the transverse interganglionic commissures) forms along the bottom of the nervous fold. This secondary median fold is very inconspicuous and slightly developed in the Cockroach, so that the transverse commissures between the developing ganglia are mainly contributed by the cellular substance of the lateral nervous band. The brain is formed out of two epiblastic thickenings which occupy shallow depressions. The so-called inner neurilemma, which surrounds the ventral nerve-cord, is developed as follows:—Along the ventral nerve-cord, and between its lateral halves, a small solid cellular band (fig. [110], Ch) is developed out of the mesoblastic diaphragm described above. This grows round the ventral nerve-cord on all sides (fig. 112, N′), passing also inwards between the central fibrillar tract and the outer cellular layer, and thus forming the thin membrane which invests the central nervous mass (fig. 112, N″). The above-mentioned solid mesoblastic band, which exists for a very short time only, may perhaps be homologised with the chorda dorsalis of Vertebrates, and the chorda of the higher Worms, since in these types also the chorda forms a solid cellular band of meso-hypoblastic origin, lying between the nervous system and the hypoblast. The peripheral nerves arise as direct prolongations of the fibrillar substance of the nerve-cord.
Fig. 113.—Alimentary Canal of Embryo of B. germanica. Copied from Rathke, loc. cit., but differently lettered. st, stomodæum, al-ready divided into œsophagus, crop, and gizzard; m, mesenteron; pr, proctodæum, with Malpighian tubules (removed on the right side). × 12.
Alimentary Canal.—The epithelium of the mesenteron is formed out of the hypoblast, whose cells assume a cubical form and gradually absorb the yolk. The epithelium of the stomodæum and proctodæum is derived, however, from two epiblastic involutions at the fore and hind ends of the embryo. The muscular coat of the alimentary canal is contributed by the splanchnic layer of the mesoblast. The mesenteron in an early stage of development appears as an oval sac of greenish colour (fig. 113), faintly seen through the body-wall. The cæcal tubes are extensions of the mesenteron, the Malpighian tubules of the proctodæum. The epiblastic invaginations may be recognised in all stages of growth by their chitinous lining and layer of chitinogenous cells, continuous with the similar layers in the external integument.
Tracheal System.—Tubular infoldings of the epiblast, forming at regular intervals along the sides of the embryo and projecting into the somatic mesoblast, give rise to the paired tracheal tubes, which are at first simple and distinct from one another.[182]