Embryonic Development

FIGURE 20.2. Stages in cleavage and blastoderm formation in egg of Dacus tryoni (Diptera). (A) Frontal section through anterior end during 6th division; (B) transverse section after 8th division; (C) transverse section after 12th division; (D) transverse section during 13th division; (E) transverse section at syncytial blastoderm stage; and (F) frontal section through posterior end after formation of uniform cellular blastoderm. [After D. T. Anderson, 1972b, The development of holometabolous insects, in: Developmental Systems: Insects, Vol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic Press Ltd., and the author.]

extra-embryonic ectoderm from which the extra-embryonic membranes later differentiate (Figure 20.3). For more than a century, attempts have been made to explain how the body pattern of an insect is determined. Following the classic experiments of the German embryologist Seidel in the late 1920s, it was widely believed that differentiation was controlled by two centers (Counce, 1973; Heming, 2003). As energids move toward the posterior end of the egg, they interact with a so-called "activation center" (Figure 20.1), and differentiation subsequently occurs. Seidel's experiments showed that neither an energid nor the activation center alone could stimulate differentiation. It was presumed that the center is caused to release an unidentified chemical that diffuses anteriorly. This diffusion is seen morphologically as a clearing and slight contraction of the yolk. As the chemical reaches

FIGURE 20.3. Diagrammatic transverse (A) and sagittal (B) sections of egg of Pontania (Hymenoptera) to show differentiation of blastoderm into embryonic primordium and extra-embryonic ectoderm. Note also the germ (pole) cells at the posterior end. [After D. T. Anderson, 1972b, The development of holometabolous insects, in: Developmental Systems: Insects, Vol. I (S. J. Counce and C. H. Waddington, eds.). By permission of Academic Press Ltd., and the author.]

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.

Get My Free Ebook


Post a comment