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620 some remain in the yolk as vitellophages that supply nutrients to the embryo. Posteriorly moving energids receive a signal at the activation center, which stimulates differentiation of part of the blastoderm into the embryonic primordium. Differentiation begins at a predetermined site, the differentiation (commitment) center, located in the region of the future prothorax. The embryonic primordium of exopterygotes is usually small and grows by aggregation and proliferation of cells, whereas that of most endopterygotes is large to permit rapid tissue differentiation and embryonic growth. Elongation and differentiation of the primordium (now known as the germ band) occur, and externally segmentation and appendage formation are obvious; internally somites form and mesoderm differentiates. Simultaneously, in embryos of most species the amnion and serosa develop from proliferating extra-embryonic cells at the margins of the germ band. Anatrepsis, movement of the germ band into the yolk core, occurs in eggs of most exopterygotes at this time.

At the end of germ band formation the amnion and serosa fuse, then break in the head region, and the combination rolls back dorsally over the yolk which is left covered by only the amnion (provisional dorsal closure). Katatrepsis now takes place in eggs with immersed germ bands, so that the embryo is returned to the yolk surface with its head facing the anterior pole of the egg. Embryonic ectoderm now extends around the yolk to replace the amnion (definitive dorsal closure).

Paired segmental appendages develop from evaginations of the embryonic ectoderm but may become reduced or disappear. Shortly after definitive dorsal closure, the embryonic ectoderm differentiates into epidermis and secretes a cuticle. Specific epidermal cells differentiate into external sensilla and eyes, and in some species form imaginal discs and histoblasts. Invaginations of the ectoderm give rise to the endoskeleton, tracheal system, salivary glands, corpora allata, molt glands, exocrine glands, and, in females, the vagina and spermatheca, in males, the ejaculatory duct and ectadenes. The foregut and hindgut develop from ectodermal invaginations at the anterior and posterior ends, respectively, of the gastral groove. The midgut is formed from anterior and posterior midgut rudiments that grow toward each other and on meeting extend dorsolaterally to enclose the yolk. The central nervous system arises from neuroblasts in the midventral line. The stomatogastric nervous system develops from evaginations in the roof of the stomodeum.

The heart, aorta, septa, muscle, fat body, paired genital ducts, and mesadenes are mesodermal derivatives. Gonads arise from primordial germ cells that become enclosed in mesoderm.

Parthenogenesis, the development of unfertilized eggs, may be ameiotic (no meiosis in oocyte nucleus) or meiotic (meiosis is followed by nuclear fusion), both of which result in diploid (female) offspring, or haploid (meiosis is not followed by nuclear fusion) from which males arise.

Polyembryony, the formation of more than one embryo in a single, small, yolkless egg, is restricted to a few parasitic or viviparous Hymenoptera and Strepsiptera.

Viviparity occurs in several forms. Ovoviviparity is retention of yolky eggs in the genital tract. In true viviparity developing offspring receive their nourishment directly from the mother. In pseudoplacental viviparity the follicle cells and embryonic membranes become closely apposed, and nourishment appears to be derived largely from the degeneration of follicle cells and from trophocytes. In adenotrophic viviparity, eggs are yolky, but larvae are retained in the uterus and feed on secretions of the accessory glands. Hemocoelic viviparity is where embryos receive nutrients directly from the hemolymph.

Paedogenesis is precocious sexual maturation of juvenile stages and is normally associated with parthenogenesis and viviparity.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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