THE DIVISION OF THE PROTOPLAST (59 



it is (tarried inward toward the cciitcr. The centripetal movement of 

 fluid along the rays is somehow compensated by a gradual outwaid 

 movement of other materials. There is evidence that the fluid rays 

 contain oriented ''structure proteins." The precise origin of the spindle 

 portion of the achromatic figure is more difficult to determine. Both 

 the karyolymph and some cytoplasmic component between the diverging 

 centrosomes evidently develop an orientation revealed b}^ the appearance 

 of " fibers " on fixation, but the manner in which they share in the develop- 

 ment of the spindle is not yet clear. 



In the anaphase and telophase the chromosomes behave as already 

 described for plants, the chromosomes mo\'ing to the poles where they 

 reorganize the two daughter nuclei. The equator which they have left 

 ordinarily shows no conspicuous change, though a little refractive and 

 stainable material may often accumulate there. The asters, near which 

 the daughter nuclei lie, remain conspicuous until after c,ytokinesis, a 

 process in which they appear to play a major role. 



Cytokinesis. — Typical cytokinesis in animals, like that in higher 

 plants, involves the achromatic figure, but it does so in a very different 

 manner. It invoh'es also a special series of changes at the cell membrane, 

 these acting with the internal forces to produce the cleavage furrow 

 which divides the cytosome. 



The large eggs of echinoderms, amphibians, and certain other animals 

 are particularly well suited to studies of the factors responsible for cell 

 cleavage. With the completion of the achromatic figure, the echinoderm 

 egg becomes noticeably elongated, and this is correlated with an enlarge- 

 ment of the two elastic, semisolid asters. It is also found that the cortical 

 plasmagel becomes firmer in consistency just before the furrow appears 

 and remains so during its inward growth. That this gelation of the 

 protoplasm is a major factor in producing the furrow is indicated by 

 the results of treatments causing a return to the sol state. Thus if the 

 asters in a cleaving egg are liquefied by stirring with a micro-needle, 

 the furrow developing between them disappears. Similarly, when the 

 rigidity of the cortical plasmagel is reduced by hydrostatic pressure, the 

 furrow ceases to grow inward or even recedes, depending upon the degree 

 of solation; furthermore, when the pressure is removed, gelation occurs 

 once more and the inward growth of the furrow is resumed. It is believed 

 b\- some investigators that the gelation produces its cleaving effect by 

 exerting a contractile tension in the equatorial plane of the cell, since 

 gelation in certain other colloidal systems is known to produce such 

 forces. 



These changes AAdthin the cell are correlated with alterations at its 

 surface. By observing the movements of small particles adhering to the 

 surface membrane of an egg beginning its cleavage, it can be established 



