118 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



of such evidence it is advisable to retain the simpler theory, namely, 

 that the cell in furrowing, as it does in amoeboid movement, derives 

 mechanical energy entirely from the contractility of the plasmagel system. 

 Following the contraction of the broad central zone, which causes the 

 cell to elongate, the contracting part of the plasmagel becomes more 

 narrowly restricted to the equator (fig. 6C). This leads to the appear- 

 ance of a definitive furrow, as is likewise postulated in the surface ex- 

 pansion theory. Thus the area of passive surface stretching also becomes 

 greater, so that now the sides of the prospective blastomeres near and 

 even in the developing furrow become involved. Moreover, as the furrow 



Fig. 6. Concept of the gel contraction mechanism of cytokinesis. Light shading 

 indicates sol; medium shading = gel, contracting moderately; dark shading = gel, 

 contracting strongly. A — anaphase; B. C and D — early, middle and late telophase. 



deepens, new plasmagel is drawn into an operative position along the 

 walls of the furrow. Thus in a well-developed furrow there are two fairly 

 broad 'annuli' of newly mobilized plasmagel constituting the walls of 

 the deepening furrow. This newly mobilized plasmagel can now contract 

 and complete the work of cleavage. Meanwhile, the plasmagel at the very 

 trough of the furrow, having expended its contractile energy, is free to 

 undergo solation. This permits the cell membrane to fuse at the division 

 axis, thus finally separating the daughter blastomeres (24). 



KINETICS OF PROTOPLASMIC CONTRACTILITY 



Earlier Evidence. Since protoplasmic gelations are endothermic, we 

 must look for a basic metabolic i)attern by which the cell provides energy 



