The ventral plate occupies, as I have explained, the future ventral surface of the Insect, and here only at first both the embryonic membranes are to be met with. On the sides and above the yolk is invested by the serous envelope alone. The ventral plate, however, gradually extends upwards upon the sides of the egg, in the directions of the arrows (fig. 107), and finally closes upon the dorsal surface of the embryo, so as completely to invest the whole yolk. Every segment of the embryo shows at a certain stage numerous clusters of spherical granules, which according to Patten (loc. cit.) are composed of urates (fig. 111, _S_). We shall now proceed to consider the development of the several organs of the Cockroach. [Illustration: Fig. 111.--Transverse section of older Embryo of _B. germanica_ (abdomen). _E_, Epiblast; _H_, hypoblast; _Ht_, heart; _G_, reproductive organs; _S_, spherical granules.] _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. [Illustration: 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.]