OOTOBEB 14, 1910] 



SCIENCE 



499 



wards the poles and terminating in the 

 division of the cytoplasm of the cell. 



I have observed in the cells of Zygnema 

 about to divide a remarkable condensation 

 of potassium in the plane of division. In 

 the "resting" cell of this Alga the potas- 

 sium is, as a rule, more abundant in the 

 cytoplasm near the transverse walls of the 

 thread, and only traces of the element are 

 to be found along the line of future divi- 

 sion of the cell. But immediately after 

 division has taken place the potassium is 

 concentrated in the plane of division. This 

 would seem to indicate that surface tension 

 in the plane of division is, as postulated by 

 the deduction from the Gibbs-Thomson 

 principle, lower than it is on the longi- 

 tudinal surface, and lower, especially, than 

 it is on the previously formed transverse 

 septa of the thread. 



One must not, however, draw from this 

 the conclusion that in all dividing cells sur- 

 face tension is lower in the plane of division 

 than it is elsewhere on the surface of the 

 dividing structure. All that it means is 

 that in the dividing cell of Zygnema the 

 condition already exists along the plane of 

 division, which subsequently makes for low 

 surface tension in the cell membrane imme- 

 diately adjacent to each transverse septum 

 in the confervoid thread. If the evidence 

 of low surface tension vanished immedi- 

 ately after division was complete, then it 

 might be held that it detei-mined the divi- 

 sion. As it is, the low surface tension in 

 this case is the result and not the cause of 

 the division. 



This conclusion is corroborated by the 

 resiilts of observations on the cells of the 

 ovules of Lilium and Tulipa. The potas- 

 siiun salts in these are found condensed in 

 minute masses throughout the cytoplasm. 

 When division is about to begin the salts 

 are shifted to the peripheral zone of the 

 cytoplasm, and when the nuclear membrane 



disappears not a trace of potassium is now 

 found in the neighborhood of the free chro- 

 mosomes, a condition which continues till 

 after nuclear division is complete. The 

 absence of potassium, the most abundant 

 basic element in the cytoplaism, would indi- 

 cate that soaps are not present, and appro- 

 priate treatment of such cells, hardened in 

 formaline only, with scarlet red demon- 

 strates that fats, including lecithins, are 

 absent also. This would seem to show that 

 high instead of low surface tension prevails 

 about the nucleus during division. During 

 the "resting" condition of the nucleus this 

 high tension is maintained, for, except in 

 very rare cases, and these of doubtful char- 

 acter, there is no condensation of inorganic 

 salts in the neighborhood or on the surface 

 of the nuclear membrane. It is also to be 

 noted that the nucleus, with exceptions, the 

 majority of which are found in the proto- 

 zoa, is of spherical shape, which also postu- 

 lates that high surface tension obtains 

 either in the cytoplasmic layer about the 

 nucleus or in the nuclear membrane itself. 

 It may also be suggested that high surface 

 tension, and not the physical impermeabil- 

 ity of the nuclear membrane, is the reason 

 why the nucleus is, as I have often stated, 

 wholly free from inorganic constituents. 



It does not follow from all this that sur- 

 face tension has nothing to do with cell 

 division. If, as Brailsford Robertson holds, 

 surface tension is lowered in the plane of 

 division, then the internal streaming move- 

 ment of the cytoplasm of each half of the 

 cell should be towards that plane, and, in 

 consequence, not separation, but fusion of 

 the two halves, would result. The lipoids 

 and soaps would indeed spread superfi- 

 cially on the two parts from the equatorial 

 plane towards the two poles, and, according 

 to the Gibbs-Thomson principle, thej^ would 

 not distribute themselves through the cyto- 

 plasm in the plane of division, except as a 



