Cleavage and Blastoderm Formation

As it moves toward the center of an egg after fusion, the zygote nucleus begins to divide mitotically. The first division occurs at a predetermined site, the cleavage center (Figure 20.1), located in the future head region, which cannot be recognized morphologically but which appears to become activated either when sperm enter an egg or when an egg is laid. Early divisions are synchronous, and as nuclei are formed and migrate

FIGURE 20.1. Positions of cleavage center, activation center, and differentiation center in eggs of Platycnemis (Odonata). [After D. Bodenstein, 1953, Embryonic development, in: Insect Physiology (K. D. Roeder, ed.). Copyright @ 1953, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.]

through the yolk toward the periplasm, each becomes surrounded by an island of cytoplasm (Figure 20.2A). Each nucleus and its surrounding cytoplasm are known as a cleavage en-ergid. In eggs of endopterygotes and possibly exopterygotes, but not those of apterygotes, the energids remain interconnected by means of fine cytoplasmic bridges.

The rate at which nuclei migrate to the yolk surface and the method of colonization are varied. In eggs of some species nuclei appear in the periplasm as early as the 64-energid state (after six divisions); in others, nuclei are not seen in the periplasm until the 1024-energid stage. In eggs of most endopterygotes and in those of paleopteran and hemipteroid exopterygotes, the periplasm is invaded uniformly by the energids. However, in eggs of orthopteroid insects the periplasm at the posterior pole of the egg receives energids first, after which there is progressive colonization of the more anterior regions.

In eggs of most insects not all cleavage energids migrate to the periphery but continue to divide within the yolk to form primary vitellophages, so-called because in most species they become phagocytic cells whose function is to digest the yolk (Figure 20.2B). In eggs of Lepidoptera, Diptera, and some orthopteroid insects, however, all of the energids migrate to the periplasm and only later do some of their progeny move back into the yolk as secondary vitellophages (Figure 20.2F). Secondary vitellophages are also produced in eggs of other insects to supplement the number of primary vitellophages. So-called tertiary vitellophages are produced in eggs of some cyclorrhaph Diptera and apocritan Hymenoptera from the anterior and posterior midgut rudiments.

After their arrival at the periplasm, the energids continue to divide, often synchronously, until the nuclei become closely packed (the syncytial blastoderm stage), after which cell membranes form by radial infolding, then tangential expansion of the original egg plasmalemma (the uniform blastoderm stage) (Figure 20.2C-F). From the resulting monolayer of cells develop all of the cells of the larval body, except in a few species where vitellophages or yolk cells contribute to the formation of the midgut (Section 7.4).

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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