. Principles of modern biology. Biology. Responsiveness in Single Cells - 199. Fig. 11-10. Cleavage divisions, as seen in the egg of a sea urchin (Arbacia punctuhfa). 1, undivided egg; 2, egg dividing, furrow well started; 3 and 4, two-cell and four-cell stages; 5-8, progressively older blastula stages. Note that finally the cells are so small and numerous that it is difficult to distinguish them individually. (Courtesy of Ethel Browne Harvey.) clues as to the nature of the force that enables the furrow to cleave the cell. Some minutes before the furrowing starts, a sur- face layer of protopla
. Principles of modern biology. Biology. Responsiveness in Single Cells - 199. Fig. 11-10. Cleavage divisions, as seen in the egg of a sea urchin (Arbacia punctuhfa). 1, undivided egg; 2, egg dividing, furrow well started; 3 and 4, two-cell and four-cell stages; 5-8, progressively older blastula stages. Note that finally the cells are so small and numerous that it is difficult to distinguish them individually. (Courtesy of Ethel Browne Harvey.) clues as to the nature of the force that enables the furrow to cleave the cell. Some minutes before the furrowing starts, a sur- face layer of protoplasm, where the furrow is about to form, suddenly solidifies into a very firm gel. This girdle of gelated proto- plasm is only about 5 microns thick, but it appears to contract forcibly, pinching the egg in two. If the cleavage girdle is prevented from gelating—by mechanically agitating the protoplasm with a microneedle, or by a va- riety of other means—the furrow does not form. Or if a liquefication of the protoplasm of the cleavage girdle is induced after the furrowing has started, the furrow gradually recedes. Secretion. Diffusional and osmotic ex- changes between the cell and its environment occur on a spontaneous basis; that is, they do not require any expenditure of energy. But in some cases the cell expends energy in forc- ing substances to pass across its living mem- branes, and such responses are called secre- tions. In multicellular organisms, the effectors of secretion are glands. Excitation of a salivary gland, for example, can be detected when- ever its motor nerve is stimulated. Action currents from the secretory cells are dis- charged so long as the motor nerve is stimu- lated, and saliva continues to flow from the gland during the period of excitation. Among unicellular organisms, the most familiar secretional effectors are the gastric and contractile vacuoles. Food in the gastric vacuole of an amoeba, for example, excites the cell to secrete digestive enzymes
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