6 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



fiber ihre immittelbare UmgebuHg zu orientiereu und demgemass ein- 

 zurichten." (p. 342.) 



(4) Jioux's theory of a compromise between the tendency immanent in 

 the nucleus and the tendency due to the form of the protoplasmic mass. 

 — Roux holds that the spiudle places itself in one of the positions of 

 stable equilibrium in relation to the protoplasmic mass, — therefore, at 

 least generally, in the longest axis of the protoplasmic mass, though 

 sometimes at right angles to that axis, the factor that decides which of 

 these positions shall be taken being an immanent tendency in the nu- 

 cleus to divide iu a certain direction. 



" Richtiger ist es zu sagen : Die Kernspindel der Furchungszellen 

 stellt sich in die, resp. iu eine Richtung festesten Gleichgewichtes der 

 tractiven Einzelwirkungen der Protoplasmamasse. Diese Richtung 

 entspricht ilberwiegeud haufig auuahernd oder ganz der grossten durch 

 den Mittelpunkt der Protoplasmamasse geheuden Dimension. 



" Diese Richtung des Gleichgewichtes wird aber uicht vollkommen 

 vom Protoplasma allein bestimmt, sondern sie kauu, wie ich bereits 1884 

 und 1885 auf Grund von Experimenten erschlossen habe, von der Lage der 

 immanenten Teilungsrichtuug des Kernes zu den Hauptrichtungeu des 

 Protoplasmakorpers abhangig sein ; denn ich erhieltbei symmetrisch ge- 

 stalteten ' linsenformig ' deformirten, rait den grossten Flache senkrecht 

 stehenden Froscheiern zwei Pradictionsrichtungen der Spindeleinstel- 

 luug : die Richtung der grossten und der kleinsten durch den Massenmit- 

 telpunkt geheuden Dimension, ei'stere allerdings wieder die Uberwiegend 

 haufige." (Roux, '94, p. 152.) 



It is to be noted that this theory does not attempt to give any rule 

 by which the position of the spiudle is necessarily determined ; the ten- 

 dency of the nucleus is simply " immaneut," and its factors unknown. 



In addition to these four well characterized theories, a number of 

 less definite or partial views have been set forth, — some proposing fac- 

 tors which may influence, though not alone determine, the position of 

 the spindle. A number of the more important of these will be 

 mentioned. 



(5) Heidenhain's prohletn of a definite angle of rotation ("Prob- 

 lem der gesetzmassigen Drehungswinkel "). — Heidenhain ('94, p. 719) 

 thinks it probable, or at least possible, that careful investigation will 

 show that in a given tissue the position which the spindle takes at the 

 time of division is a result of its rotation through a definite angle, de- 

 terminable for the given tissue, after the first formation of the spindle 

 by the separation of the asters. This separation of the asters is held 



