198 - The Cell 



nucleus 

 food vacuole 



contractile 

 vacuole 



region of 

 sofatlon 



region of gelation 

 hyaline fluid. 



Fig. 11-9. Amoeboid movement involves a continuous series of gelation and solation 

 reactions (see text). 



of the amoeba, the plasmagel is solating 

 forming new plasmasol that adds itself to 

 the forward-flowing stream. There are, how- 

 ever, a number of unknown factors as to the 

 mechanism of amoeboid movement. Virtu- 

 ally nothing is known about the metabolic 

 reactions that determine the soiling and 

 gelling of the protoplasm, nor about how 

 these processes are coordinated in the differ- 

 ent parts of the cell. 



Cyclosis. Quite a variety of plant and ani- 

 mal cells exhibit cyclosis, a rotational stream- 

 ing of the protoplasm that does not effect 

 any change in the external form of the cell. 

 In certain plants, such as Eluded and Nitclla 

 (Fig. 13-2), the cells display an exceptionally 

 rapid cyclosis, and these cells are favorable 

 for a study of the movement. 



Cyclosis, like amoeboid movement, seems 

 to depend upon alternating processes of sola- 

 tion and gelation, but just how these reac- 

 tions produce streaming is hard to under- 

 stand. In Nitella, the streaming can be 

 stopped and started by a variety of stimuli, 

 and each successful stimulation produces a 

 typical wave of excitation, attended by an 

 action potential. Cyclosis serves to accel- 

 erate the distribution of substances from one 

 part of the cell to another, or from one cell 

 to neighboring cells — in multicellular organ- 

 isms. In many cells, cyclosis goes on more or 

 less continuously, even in the absence of any 



apparent external stimulation. However, such 

 a continuous cyclosis probably depends upon 

 a series of internal stimuli, which arise from 

 rhythmically recurring processes in the me- 

 tabolism of the cell. 



Cleavage. Cleavage is the pinching move- 

 ment by which an animal cell cuts itself into 

 daughter cells, and this movement has been 

 studied most extensively in dividing eggs 

 (Fig. 11-10). These large cells are very favor- 

 able because they cleave on a regular sched- 

 ule, following the time when the sperm make 

 contact with the eggs. 



When a sperm comes into contact with an 

 egg, a definite excitation arises at the con- 

 tact point, and this excitation sweeps over 

 the entire egg surface, visibly changing the 

 protoplasmic structure. The original stimu- 

 lus initiates a long series of responses that in- 

 clude the lifting of the fertilization mem- 

 brane (p. 273), the penetration of the sperm 

 head, the approach and fusion of the gamete 

 nuclei, the divisions of the zygote nucleus, 

 and the successive cleavages of the one cell 

 into many (Fig. 11-10). 



The cleavage furrow (Fig. 11-10), which 

 cuts through the cell, first appears as a shal- 

 low groove encircling the egg. Then gradu- 

 ally the furrow deepens, and in a few min- 

 utes, it passes through the center of the egg 

 and completes the division of the cytoplasm. 



Recent evidence (p. 48) provides several 



