190 The Structure of Protoplasm 



Endoplasm then meets endoplasm in a broad equatorial area. 

 With the continued deepening of the furrow, by the contraction of 

 the gel layer at the bottom of the groove, the endoplasm is divided 

 and each half is incorporated into the endoplasmic layer of the 

 daughter cells. At the bottom of the groove, gel layer then meets gel 

 layer to form a stalk or band which connects the two daughter cells. 

 This band usually persists until the cells migrate away from one 

 another and stretch it to a thinner and thinner strand which finally 

 breaks. Endoplasm and interchromosomal material mix after the 

 former is divided and included in the daughter cells. 



The observable steps in cleavage indicate that the interchromo- 

 somal material is less viscous than the endoplasm and the latter 

 less viscous than the gel layer or gel band which contracts. The 

 constriction band is presumably either a thickened or more viscous 

 band of the gel layer, which develops around the equator after the 

 cell has become bilateral. The cell becomes bilateral, or approxi- 

 mately so, when the chromosomes become arranged in the median 

 plane. It has already been noted that cells flatten in the equatorial 

 region as the chromosomes begin to move toward the poles and that 

 the constriction furrow begins after they have reached the pole. The 

 factors which produce the constriction band are unknown, but it 

 seems probable that the new bilateral set-up may have something 

 to do with it. Some metabolic products of chromosomes, centrosomes, 

 and interchromosomal material may be involved. 



During cleavage of the fibroblast there are no indications of any 

 cytoplasmic currents or flow. As the endoplasm is bent inward by 

 the contraction of the gel band, fat globules in the neighborhood of 

 and at the bottom of the furrow are bent in; those at a distance are 

 scarcely disturbed at all. 



My theory of cell cleavage is based on the idea that gels auto- 

 matically exert contractile tension and that the contractile tension 

 of the gel layer is increased in the median equatorial plane by an 

 increase of its thickness or of its viscosity, or of both in this region. 

 This increase automatically leads to the contraction which pinches 

 the cell in two. The existence of a superficial gel layer seems to be 

 a fact. The automatic contractility, which it exerts because it is in 

 the gel state, has already been considered. That the contractile 

 tension varies with the thickness and with the viscosity seems like 

 a probable assumption. Is there any direct evidence for the develop- 

 ment of an equatorial band? The gel layer of fibroblasts, on which 

 most of my observations are based, is rather thin, but one can some- 



