STUDY OF CELL MECHANICS 467 



there is an osmotic change in the protoplasm just beneath that 

 portion of the ectoplasmic layer which swells later. In other 

 words that preceding the active changes of form which the pro- 

 toplasm undergoes, there are great changes in the osmotic rela- 

 tions of this side of the egg, as is shown by the swelling of the 

 ectoplasmic layer. If we may safely compare the experiments 

 of Goldschmidt and Popoff with our eggs, we should say that 

 the movement was preceded by an increase in the density of one 

 side of the egg. 4 This would carry with it, of course, a differ- 

 ence in the permeability of different parts of the egg. 5 



To what was this change in density due? Three possible 

 sources could be suggested. First the aster, second the nucleus, 

 and third the cytoplasm. 



The aster. The movement of the aster was first considered 

 as the cause of the movement, because the distance which the 

 aster moved through the egg was a measure of the intensity 



4 This is precisely what Butschli ('92) has maintained, and with him, a num- 

 ber of other investigators. Among the latest of these is McClendon ('10, '12). 

 Robertson ('09, '13) has taken issue with McClendon and claims that, "In cell 

 division the cleavage furrow is a region of low superficial tension, the poles of 

 the egg regions of high superficial tension. Butschli and McClendon's view to 

 the contrary is shown to involve a contradiction of the laws of molecular attrac- 

 tion of the liquids." (Robertson, Arch. f. Entw. Mech., vol. 35, p. 707). Rob- 

 ertson's views are based upon certain experiments with oil drops. "Drops of 

 rancid olive oil were floated upon water, and threads wetted with solutions of 

 strong bases, were laid across the drops. Division resulted owing to the equa- 

 torial diminution of surface tension resulting from the formation of soap" (Rob- 

 ertson, I.e., p. 692). In describing the behavior of the oil drops, he distinctly 

 points out that there is a superficial flow of substance away from the equator 

 towards the poles of the egg: "and violent streaming motions occur at the sur- 

 face away from the thread and towards the opposite poles of the drop." (Arch, 

 f. Entw. Mech., vol. 27, p. 30.) This is exactly the reverse of the condition 

 found in the living egg, by a great number of investigators, including myself. 

 Clearly, the conditions within the egg, are quite different from those in the oil 

 drops with which Robertson experimented. This flow of substance towards 

 the cleavage plane involves an increase of surface tension in this region, as Rob- 

 ertson points out, "for an increase in surface tension at the equator, such as 

 Butschli imagines to occur would result in a streaming of material towards the 

 equator" (p. 29, Arch. f. Entw. Mech., vol. 27). Robertson made the error of 

 comparing the conditions within the oil drop (a homogenous body) with that of 

 the living egg. 



5 See Lillie, '16. 



