. Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching. Embryology -- Insects; Embryology -- Myriapoda. 404 EMBRYOLOGY OF INSECTS AND MYRIAPODS The Alimentary Canal.—The origin of the entodermal cells has already been mentioned. They form a thin layer on the ventral side of the egg immediately outside the yolk (Figs. 340^, 346, ent). This layer spreads, apparently by a process of stretching, over the entire yolk as a thin-walled sac before the formation of the individual coelomic sacs. The yolk cells (trophoc
. Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching. Embryology -- Insects; Embryology -- Myriapoda. 404 EMBRYOLOGY OF INSECTS AND MYRIAPODS The Alimentary Canal.—The origin of the entodermal cells has already been mentioned. They form a thin layer on the ventral side of the egg immediately outside the yolk (Figs. 340^, 346, ent). This layer spreads, apparently by a process of stretching, over the entire yolk as a thin-walled sac before the formation of the individual coelomic sacs. The yolk cells (trophocytes, or vitellophags) are similar to entoderm cells in physiological and morphological characters, the nuclei being usually larger, often irregular in shape, with one or more nucleoli. The yolk *%®ais—br. Fig. 360.—Scolopendra cingulata. Sagittal section of the stomodaeum. {br) Brain. ifg) Anlage of frontal ganglion. (Zr) Labrum. {mus) Muscle, ijn) Recurrent nerve. {sin) Sinus. {Adapted from Heymons.) cells in the egg center break down by the time segmentation of the germ band is completed, and soon thereafter the yolk pyramids also dis- integrate, the yolk being reduced to a homogeneous mass by the time the lateral halves of the germ band separate from each other (Fig. 341^4). By this time or soon afterward the yolk cells become more uniformly dis- tributed through the yolk (Fig. 348) and are then characterized by their much larger nuclei, preliminary to their degeneration. The entodermal cells, which now form a sac-Uke layer around the yolk, take over the function of reducing the yolk. Meanwhile the stomodaeum and proc- todaeum have formed as simple sac-like invaginations (Fig. 360), the former arising shortly before the latter. The dorsal wall of the stomo-. 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 origina
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