. Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching. Embryology -- Insects; Embryology -- Myriapoda. 66 EMBRYOLOGY OF INSECTS AND MYRIAPODS blast; the inner one, the entoderm, or hypoblast; the two constituting the primary germinal layers of the embryo. Excepting among the lowest forms of the Metozoa, there is subsequently formed between these a third layer, the mesoderm (mesoblast) (Figs. 39, 40). Gastrulation as described above occurs in a number of different sec- tions of the animal series; but in many
. Embryology of insects and myriapods; the developmental history of insects, centipedes, and millepedes from egg desposition [!] to hatching. Embryology -- Insects; Embryology -- Myriapoda. 66 EMBRYOLOGY OF INSECTS AND MYRIAPODS blast; the inner one, the entoderm, or hypoblast; the two constituting the primary germinal layers of the embryo. Excepting among the lowest forms of the Metozoa, there is subsequently formed between these a third layer, the mesoderm (mesoblast) (Figs. 39, 40). Gastrulation as described above occurs in a number of different sec- tions of the animal series; but in many instances, owing to a relatively large amount of yolk present in the eggs of some animals, it may become modified in various ways so that it is doubtful if in some of these cases true gastrulation occurs. Cases in which the blastula is a hollow ball of cells, one hemisphere apparently dimpled into the other (Fig. 40), are represented by the eggs. bp Fig. 39. Fig. 40. Fig. 39.—Section. Mesoderm formation {mes). (bp) Blastopore, (ect) Ectoderm. (ent) Entoderm. Fig. 40.—Sagitta (arrowworm). Folds developing from the bottom of the gastrula giving rise to the definitive mid-gut (enf) and the coelomic diverticula (cod), (ect) Ecto- derm, (ejit) Entoderm, (som. m) Somatic mesoderm, (splm) Splanchnic mesoderm. (From Hertwig.) of the arrowworms (Sagitta), many moUusks (Paludina, etc.), sponges, coelenterates, worms, echinoderms, and Amphioxus. In this process of invagination (emboly) the outer layer forms the ectoderm; the inner layer, the entoderm. In the medusa Lucernaria where a morula is formed, the cavity being absent, the cells of the animal pole (ectoderm) grow over those of the vegetative pole (entoderm). This process of overgrowth is known as "; A modification of this type is found also in the yolk-laden eggs of birds and reptiles with discoidal cleavage wherein the germ disk overgrows the yolk. In some coelenterates a layer of cells is formed inside the b
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