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.
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).
Fig. 111.—Transverse section of older Embryo of B. germanica (abdomen). E, Epiblast; H, hypoblast; Ht, heart; G, reproductive organs; S, spherical granules.
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.