Qxx

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.]

the future prothoracic region of the embryo (the "differentiation center") (Figure 20.1), the blastoderm in this region gives a sharp twitch and becomes slightly invaginated. Blastoderm cells aggregate within this invagination and differentiate into the embryonic primordium. (Later in embryogenesis, other processes, for example, mesoderm formation and segmentation, begin at the differentiation center and spread anteriorly and posteriorly from it.)

An alternate view for the cause of embryonic differentiation is the "gradient hypothesis," which had its origins at the end of the 19th century but then fell out of favor after Seidel's pioneering work (Sander, 1984,1997; Lawrence, 1992). Essentially, the hypothesis proposes that a chemical produced at each end of an egg diffuses throughout the egg, producing two gradients of concentration (Figure 20.4). Cells within the egg then "recognize" their position within the egg by the relative concentrations of the chemical and differentiate accordingly. Initial support for the existence of chemical gradients in eggs came from experiments in which eggs either were ligatured at various distances along their length and at varied times after embryonic development began or were centrifuged, thereby disrupting the proposed gradient. Recently, the application of genetic and molecular techniques to the study of pattern development in Drosophila has given further support to the idea of gradients. Thus, a modern interpretation of Seidel's differentiation center is that it is a "commitment center"; that is, it is the point at which blastoderm cells are committed to following a particular path of differentiation by virtue of their position within the gradients (Heming, 2003).

FIGURE 20.4. Diagrammatic representation of the gradient hypothesis. A chemical produced at each end of an egg diffuses lengthwise, forming two gradients of concentration. At any point along the length of the egg, the relative concentration of the two chemicals provides positional information to cells.

Posterior

Anterior

FIGURE 20.4. Diagrammatic representation of the gradient hypothesis. A chemical produced at each end of an egg diffuses lengthwise, forming two gradients of concentration. At any point along the length of the egg, the relative concentration of the two chemicals provides positional information to cells.

anterior

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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