Embryology of insects and myriapods; Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching embryologyofinse00joha Year: 1941 OLIGOENTOMATA AND APTILOTA 181 gonads. On the dorsal edge of the developing somite (Fig. 97, cbl) are the minute cardioblasts. Of special interest are labial glandular structures, not found in ptery- gote insects unless they correspond to the embryonic esophageal gland. In Isotoma some time before the differentiation of the mesodermic somites, in the labial segment at the level of the n
Embryology of insects and myriapods; Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching embryologyofinse00joha Year: 1941 OLIGOENTOMATA AND APTILOTA 181 gonads. On the dorsal edge of the developing somite (Fig. 97, cbl) are the minute cardioblasts. Of special interest are labial glandular structures, not found in ptery- gote insects unless they correspond to the embryonic esophageal gland. In Isotoma some time before the differentiation of the mesodermic somites, in the labial segment at the level of the nerve ganglion, a little glandular vesicle (Fig. 87, hg) is in evidence. Later during the course of development the vesicle becomes flask-shaped with a slender duct leading toward but not communicating with the exterior (Fig. 98, hg). This Fig. 97.—Isotoma. Cross section of thorax, {am. ser) Amnioserosa. (cbl) Cardioblasts. (do) Dorsal organ. (ggl) Ganglion, (mge) Mid-gut ep- ithelium, {mus) Muscle, (neurp) Neuropile. {yc) Yolk cell. Fig. 98.—Isotoma. tion through head. Ocelli. (,ggl) Ganglion. Parasagittal sec- (6r) Brain, (oc) {hg) Head gland. structure is therefore not of ectodermic origin, though later an ectodermic connection with the exterior is established. According to the evidence presented by Philiptschenko (1912) these glandular structures appear to be homologous with nephrideal head glands found in certain other Arthropoda. As has already been stated, the germ cells of Isotoma were segregated in the 16- or 32-cell stage. By the time the proctodaeum is well formed, the germ cells have divided into two groups and pass from the yolk into the tissue of the visceral wall of the mesodermic somites (Fig. 99, gc) of the third and fourth abdominal segments. Later the germ cells migrate forward into the first to the third segments where the gonads form ventrad of the aUmentary canal. The gonads and their ducts develop from the visceral mesoderm, the latter in the fourth
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