. Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching. Embryology -- Insects; Embryology -- Myriapoda. 20 EMBRYOLOGY OF INSECTS AND MYRIAPODS neurp mst. eci neurg neur Fig. 22.—Section of ventral nerve cord, {dt) Daughter cells, {ect) Ecto- derm, {mat) Median nerve strand. {neur) Neuroblast, {neurg) Neural groove, {neurp) Neuropile. {From Wheeler.) entirety, therefore, first develops at each extremity and later is formed in the middle. As has already been stated, blood cells arise from cells liberated along
. Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching. Embryology -- Insects; Embryology -- Myriapoda. 20 EMBRYOLOGY OF INSECTS AND MYRIAPODS neurp mst. eci neurg neur Fig. 22.—Section of ventral nerve cord, {dt) Daughter cells, {ect) Ecto- derm, {mat) Median nerve strand. {neur) Neuroblast, {neurg) Neural groove, {neurp) Neuropile. {From Wheeler.) entirety, therefore, first develops at each extremity and later is formed in the middle. As has already been stated, blood cells arise from cells liberated along the midventral line above the inner neural ridge. By some embryologists these are regarded as coming from the inner lateral margins of the coelomic sacs adjacent to the middle strand and therefore mesodermic; other writers, Hirschler (1909) among them, say that these cells come from the middle strand itself and therefore, hke the mid-gut epithelium, are entodermic. The pericardial cells, likewise re- garded by some embryologists as mesodermic and probably derived from the middle section of the peri- cardial septum (Fig. 21, yd), are said by Hirschler (1909) to arise from those cells of the posterior mesen- teron rudiment which are not used in the formation of the posterior part of the mid-gut epithelium. The paired structure found in the pericardial septum in metameric arrangement at the sides of the dorsal blood vessel in Forficula and some other insects, and known as the paracardial cell strand, arises from the dorsal part of the coelomic sacs. Nervous System.—Shortly before the appearance of the stomodaeal and proctodaeal invag- inations the neural groove (Fig. 16, neurg) forms along the mid- ventral longitudinal line, extend- ing at a little later stage from near the stomodaeum to the procto- daeum. On each side of the groove outwardly is an outer neural ridge which later develops into the paired nerve strands. The outer neural ridges become segmented. The ectoderm adja
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