. Zoology for high schools and colleges. Zoology. .-lo. I'ig. 291.—Male Pexual apparatus of a bark-beetle. 8l^ vas deterene ; Jw, resile ; bl, seminal vesicle ; ag^ ductus ejaculatorius.—After Graber. Fig, 292.—<, testis; «, vas deferens; g. seminal vesicle of Acheta campestris.—After 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 high schools and colleges. Zoology. .-lo. I'ig. 291.—Male Pexual apparatus of a bark-beetle. 8l^ vas deterene ; Jw, resile ; bl, seminal vesicle ; ag^ ductus ejaculatorius.—After Graber. Fig, 292.—<, testis; «, vas deferens; g. seminal vesicle of Acheta campestris.—After 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 land. 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 Fig. of Sphinx membrane (called amnion, but not 'iilirB'irmlm'' homologous with that of Vertebrates) brane;am., amnion; A,outer, peels off frOm the primitive band lust »i, inner germ-layer. ^ r ^ 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. 293-298, the. 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, H. Holt and Company
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