Each of the two mesoblastic bands which lie right and left of the germinal groove divides into many successive somites, and each of these becomes hollow. Every such somite consists of an inner (dorsal) one-layered and an outer (ventral) many-layered wall, the latter being in contact with the epiblast. The cavities of all the somites unite to form a common cavity, the _cœlom_ or perivisceral space of the Cockroach. The cœlom, like the cavities in which it originates, is bounded by two layers of mesoblast--an inner, the so-called _splanchnic_ or visceral layer, which lies on the outer side of the hypoblast, and an outer _somatic_ or parietal layer, beneath the epiblast. There are accordingly four layers in the Cockroach-embryo--viz., (1) _epiblast_, from which the integument and nervous system are developed; (2) _somatic layer of mesoblast_, mainly converted into the muscles of the body-wall; (3) _splanchnic layer of mesoblast_, yielding the muscular coat of the alimentary canal; and (4) _hypoblast_, yielding the epithelium of the mesenteron. [Illustration: Fig. 109.--Transverse sections of Embryo of _B. germanica_, with rudimentary nervous system (Oc. 4, Obj. D.D. Zeiss). _N_, nervous system; _M_, mesoblastic somites.] Scattered yolk-cells associate themselves with the mesoblast cells, so that the constituents of the mesoblast have a two-fold origin. Fig. 109 shows that the yolk-cells are large, finely granular, and provided with many (3–6) nuclei and nucleoli. They send out many branching protoplasmic threads, which connect the different cells together, and thus form a cellular network. Certain cells separate themselves from the rest, apply themselves to the walls of the somites, and form a provisional diaphragm (fig. 110, _D_) consisting of a layer of flattened cells;[181] other cells (fig. 109) pass into and through the walls of the somites, and reach their central cavity, where they increase in number and blend with the mesoblast cells. What finally becomes of them I cannot say; perhaps they form the fat-body. [181] Cf. Korotneff, Embryol. der Gryllotalpa. Zeits. f. wiss. Zool. (1885). [Illustration: Fig. 110.--Transverse section through ventral region of Embryo of _B. germanica_. The nerve-cord has by this time detached itself from the epiblast, _E_. _D_ is the temporary diaphragm; _Ch_, temporary cellular band, from which the neurilemma proceeds; _Ap_, appendages in section; _M_, mesoblast; _N_, nerve-cord. (Oc. 4. Obj. BB. Zeiss).]