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 EARLY DEVELOPMENT 43 nected by preoral commissures. The cephalic mesoderm extends forward from the somatic mesoderm bands and, in its fullest development, sur- may become excavated by cavities rounds the mouth anteriorly; it corresponding with the first an- tennae (ant), the preantennae (pren), and the labrum (Ir).' Roonwal (1939), however, is not convinced of the soundness of Snodgrass
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 EARLY DEVELOPMENT 43 nected by preoral commissures. The cephalic mesoderm extends forward from the somatic mesoderm bands and, in its fullest development, sur- may become excavated by cavities rounds the mouth anteriorly; it corresponding with the first an- tennae (ant), the preantennae (pren), and the labrum (Ir).' Roonwal (1939), however, is not convinced of the soundness of Snodgrass' interpretation on the ground that not sufficient stress is given to embryological evidence. He concludes that on developmen- tal criteria we must accept the existence of both the labral and preantennary segments in insects and these, together with the five following segments, indicate a seven-segmented nature of the in- sect head. APPENDAGES It is probable that the early ancestors of the Arthropoda had a pair of appendanges on each of the somites between the prostomi- um and the anus-bearing terminal segment. Although typically seg- mented in postembryonic life, in the embryo these appendages ap- pear as lateral or lateroventral evaginations of the body wall, simple and lobe-like, which only later secondarily may become segmented. The antennae and post- antennae were regarded by most embryologists, at least until recently, as parts homologous with the gnathal appendages and the thoracic legs. Preantennal appendages have also been described in Scolopendra (Heymons, 1901) and Carausius (Wiesmann, 1926). Even the eyes of arthropods were by some believed to be homologous with the appendages, on the basis that the stalks bearing the eyes in Crustacea are segmented and movable and that when an eye is removed under certain conditions an antenna-like structure is regenerated. Embryologists in general consider the labrum not as an appendage but as an unpaired lobe of the head that has
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