We should observe that the terms ectoderm, mesoderm, and endoderm will probably be no longer applied to the layers of the embryo when embryologists shall have decided as to the nature of the derived layers, and shall have agreed as to names for them. According to the nomenclature of Graber[75] the blastoderm differentiates into Ectoblast and Endoblast; this latter undergoing a further differentiation into Coeloblast and Myoblast. This talented embryologist gives the following table of the relations of the embryonic layers and their nomenclature, the first term of each group being the one he proposed to use:— Periblast-------Ectoblast Part of yolk cells. (Epiblast, \ (Ectoderm, outer / / blastoderm).\ layer). / / \ / Protoblast. \ / Coeloblast \ Centroblast Endoblast or--------(Endoderm in narrower \(Yolk-cells, Hypoblast, \ sense). hypoblast, inner layer. \ _Darmdrüsenblatt_. endoderm (Mesoblast of \ in part of Balfour.) \ Balfour.) Mesoderm and Myoblast (Mesoderm endoderm. of most authors). _Darmmuskelblatt_ Nussbaum considers[76] that "there are 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." {150}[Illustration: FIG. 81.—Early stages of the segmentation of a beetle (_Lina_): A, segmentation not visible, 1 day; B, segmentation of head visible; C, segmentation still more advanced, 2¼ days; PC, procephalic lobes; _g^1_, _g^2_, _g^3_, segments bearing appendages of the head; _th_, thorax; _th^1_, _th^2_, _th^3_, segments of the thorax; _a^1_, _a^2_, anterior abdominal.]