Responsiveness in Single Cells - 1 99 



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 across 

 the vacuole membrane, from the surround- 

 ing cytoplasm. This mobilization requires the 

 cell to do work, since enzyme molecules are 

 too large to penetrate the membrane spon- 

 taneously. In some way the cell manages to 

 pass the enzymes into the digestive vacuoles, 

 but exactly how this is done is problematical. 

 In stained specimens the enzyme material ap- 

 pears to accumulate on the outer surface of 

 the vacuolar membrane in the form of visible 

 granules, and these granules seem to erupt 

 through the membrane into the vacuole 

 proper. 



How the contractile vacuole functions is 

 likewise not well understood. The content 



