194 The Structure of Protoplasm 



appeared in the present experiments, which indicates that a maximum 

 rigidity is present in the equatorial region." "The role of the asters 

 in the cleavage phenomenon remains for consideration. If the 

 process which pushes the rigid walls of the furrow inward represents 

 the interpolation of plasmagel which derives from the subjacent 

 plasmasol, it is possible that the astral rays represent the lines of 

 flow whereby this translocation of materials is accomplished." 



If Marsland had used the words "pulled" and "pulls" instead of 

 "pushed" and "pushes", his interpretations would agree perfectly 

 with the idea that the equatorial girdle exerts contractile tension. 

 The question as to how much the stretched plasmogel layer of the 

 furrow is augmented by gelation of the subjacent plasmosol is a 

 difficult one to answer. Schechtman's experiments show that the 

 plasmogel is stretched, that is, the superficial part containing pigment 

 granules and dye spots. It would seem probable, however, that there 

 is some augmentation by gelation of the subjacent plasmosol. There 

 is no necessity for assuming that the plasmosol adjacent to the 

 furrow is augmented by a translocation of materials along the astral 

 rays, unless the plasmogel consists of a different sort of protoplasm 

 than that of the adjacent plasmosol, and there is no particular 

 evidence for this. 



DIVISION OF AMEBA 



Chalkley in his paper on "The Mechanism of Cytoplasmic 

 Fission in Amoeba Proteus" states that "general cell locomotion and 

 cytoplasmic division may be intimately related as to mechanism." 

 With this I entirely agree. He also states that "with the approach 

 of the daughter nuclei to the surface, the elastic strength of the 

 plasmagel layer will be sharply lessened in their immediate vicinity. 

 The greater elastic strength of the plasmagel at the equator will 

 result in the gradual constriction in this region. This will press 

 the plasmasol into these weakened areas, and polar pseudopodia will 

 protrude. With their attachment to the substrate, locomotion will 

 be initiated, and the cell will elongate. The streaming of the cyto- 

 plasm will carry the nuclei outward, and their continued activity 

 will maintain opposing polar regions of low tension in the plasmagel. 

 The locomotion mechanism posited by Mast will thus be brought into 

 play and maintained in opposite directions, and will result in the 

 continual narrowing and stretching of the equator of the cell by 

 traction of the daughter cells, until the cell is parted, and fission 

 is complete." 



