328 ON THE INTERNAL FORM . [ch. 



that of the medium in which they He seems all that is required 

 to account for their excursions; and such changes of density are 

 not only of likely occurrence during the active chemical operations 

 associated with fertilisation and division, but are in all probability 

 inseparable from the changes in viscosity which are known to 

 occur*. The movements and arrangements of the chromosomes, 

 already described, may be easily accounted for if we postulate, in 

 addition to their repulsion from the oscillating centrosomes, induced 

 oscillations in themselves such as to cause them to attract one another. 



The well-defined length of the spindle and the position of equili- 

 brium in which it comes to rest may be conceived as resultants of 

 the several mutual repulsions of the centrosomes by one another, 

 by the chromosomes or other lighter material of the equatorial plate, 

 and again by such lighter material as may have accumulated at the 

 periphery of the egg ; the first two of these will tend to lengthen the 

 spindle, the last to shorten it; and the last will especially affect its 

 position and direction. When Chambers amputated part of an 

 amphiastral egg, the remains of the amphiaster disappeared, and 

 then came into being again in a new and more symmetrical position ; 

 it or its centrosomal focus had been symmetrically repelled, we may 

 suppose, by the fresh surface. Hertwig's law that the spindle-axis 

 tends to lie in the direction of the largest mass of protoplasm, in 

 other words to point where the cell-surface lies farthest off and its 

 repulsion is least felt, may likewise find its easy explanation. 



Between these hypotheses we may choose one or other (if we 

 choose at all), according to our judgment. As Henri Poincare tells 

 us, we never know that any one physical hypothesis is true, we take 

 the simplest we can find; and this we call the guiding principle of 

 simphcity ! In this case, the hydrodynamic hypothesis is a simple 

 one; but it all rests on a hypothetic oscillation of the centrosomes, 

 which has never been witnessed. Bayliss has shewn that precisely 

 such reversible states of gelation as we have been speaking of as 



* Cf. G. Odquist, Viscositatsanderungen des Zellplasmas wahrend der ersten 

 Entwicklungsstufen des Froscheies, Arch. f. Entw. Mech. ia, pp. 610-624, 1922; 

 A. Gurwitsch, Pramissen und anstossgebende Faktoren der Furchung und 

 Zelltheilung, Arch. f. Zellforsch. n, pp. 495-548, 1909; L. V. Heilbrunn, 

 Protoplasmic viscosity-changes during mitosis, Journ. Exp. Zool. xxxiv, pp. 417-447, 

 1921 ; ibid, xliv, pp. 255-278, 1926; E. Leblond, Passage de I'etat de gel a I'etat de sol 

 dans le protoplasme vivant, C.R. Soc. Biol, lxxxii, p. 1150; of. ibid. p. 1220; etc. 



