MUSEUM OF COMPARATIVE ZOOLOGY. 323 



the nucleus ; in many cases it is the immediate cause of the division, 

 and it is no contradiction of this that nuclear division may take place 

 without an accompanying cell division, for the field of its (nucleus) in- 

 fluence must have a limit. Most clearly does the nucleus sustain a 

 causal relation to the division in those cases where it is eccentric in 

 position, inasmuch as in these cases the segmentation invariably begins 

 on the surface of the yolk nearest to the nucleus. 



The idea that the radial systems are due to centres of attraction, as 

 the author, together with others, once maintained, he is now inclined 

 to surrender, because it leaves the cause of the yolk division quite unex- 

 plained. He now maintains " that the radial arrangement of the plasm 

 around the central area is the expression of a physico-chemical altera- 

 tion of the plasm, emanating from the area, and that a gradual dimi- 

 nution of this alteration — which receives its support from the central 

 area — takes place from the central area toward the periphery." 



The assumption that the radial structure is of this nature is, in 

 Biitschli's opinion, sufficient to account for the origin of an inequality in 

 the superficial tension of the sphere in case the chemical changes tend 

 to an increase of tension.* This tension will necessarily be restored to 

 equilibrium in accordance with the physical law that the superficial tension 

 is inversely proportional to the radius of curvature. As the maximum 

 tension will occur where the activities of the two centres combine (viz. 

 in the equator f), the equilibrium will only then be restored when the 

 radius of curvature at the equator is increased, and at the poles is 

 diminished. But this is equivalent to a prolation of the sphere. The 

 chemical changes continue, and as the prolation results in removing the 

 poles of the spheroid farther from their respective central areas, and in 

 bringing the latter nearer to the equator, the difference in superficial 

 tension must become exalted. A constant increase in the superficial 

 tension at the equator will result finally in an " Einfurchung " in that 

 plane, inasmuch as by this means the force of cohesion will be consider- 

 ably diminished by its outwardly directed negative component. 



Aside from the author's confessed inability to explain fully the subse- 

 quent " Durchfurchung," one feels impelled to inquire what ground 



* The sphere obeys the fundamental laws of a fluid mass. 



t The author admits that this plane, as is evident, will be the equator only 

 under certain conditions relative to the rapidity wdth which the changes in the 

 plasm are propagated, but thinks that these conditions may well be assumed to be 

 present. On the other hand, it may be said that in Liraax the stellate figures are 

 certainly not always synchronous in their origin, — that one may have attained con- 

 siderable dimensions before the other is discernible. 



