. Zoology : for students and general readers . Zoology. Fig. 296.—Male sexual apparatus of a bark-beetle, si, vas deferens ; Ju), testis ; U, seminal Yesicle ; ag, ductus ejaculatorius.—After Graber. Fig. 397.—<, testis: v, vas deferens ; g, seminal vesicle of Acheta campestris.—Alter Gegenbaur. There is a remarkable uniformity in the mode of develop- ment of the winged insects. In general, after fertilization of the egg, a few cells appear at one end of the egg ; these multiply, forming a single layer around the egg, this layer constituting the blastoderm. This layer thickens on one side o
. Zoology : for students and general readers . Zoology. Fig. 296.—Male sexual apparatus of a bark-beetle, si, vas deferens ; Ju), testis ; U, seminal Yesicle ; ag, ductus ejaculatorius.—After Graber. Fig. 397.—<, testis: v, vas deferens ; g, seminal vesicle of Acheta campestris.—Alter Gegenbaur. There is a remarkable uniformity in the mode of develop- ment of the winged insects. In general, after fertilization of the egg, a few cells appear at one end of the egg ; these multiply, forming a single layer around the egg, this layer constituting the blastoderm. This layer thickens on one side of the egg, forming a whitish patch called the primitive streak or hand. The blastoderm molts, sloughing off an outer layer of cells, a new layer forming beneath ; the skin thus thrown off is called the serous membrane ; the second germ-layer (ectoderm) then arises, and a second membrane (called amnion, but not homologous with that of vertebrates) peels off from the primitive band just as the appendages are budding out, so that the body and appendages of the embryo insect are en- cased in the amnion as the hand and fingers are encased by a glove. As seen in the accompanying Figs. 298-303, the. Ficr. 298.—Section of Sphinx embryo, the germ immersed in the yolk ; 5, serous mem- brane; am, amnion; h, outer, ?«., inner Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Packard, A. S. (Alpheus Spring), 1839-1905. New York : Henry Holt
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
