No. 2.] OQ KINESIS. 



233 



observed by Hertvvig (5), Fol (6), Selenka (p. 87), Mark 

 (8), and others. 



For the sake of simplicity, we have proceeded thus far on 

 the assumption that the pronuclei are contemporaneous in 

 origin, and that they begin to migrate simultaneously. But the 

 conditions will agree more nearly with those generally occurring 

 in nature, if we represent them as varying considerably both in 

 the time of origin and the time of starting. The slight advan- 

 tage in respect to starting-point, which the female pronucleus 

 has over the male pronucleus, and the velocity of movement, can 

 be left out of account in considering the influences that affect 

 the course and direction of migration. 



The time of starting is the chief source of the variations which 

 we have now to consider. The difference in this respect is 

 carried to the extreme in those cases where one of the pronu- 

 clei reaches its destination before the other is ready to begin its 

 march. In such cases the earlier pronucleus, starting from any 

 point (as f or f'^ in the periphery, would 

 be carried, by centripetal attraction alone, 

 in a straight line to the centre (c). From 

 this point it would then advance along an- 

 other radius to meet the later pronucleus 

 at some point (as ;tr) ; and, after meeting, 

 the united pronuclei would move back to c. 

 The centrifugal movement of the early pro- 

 nucleus from c to X would be slower than '^'^' 

 the centripetal movement of the later pronucleus from 7,-1 to ,r, 

 since the former movement would represent the difference, and 

 the latter the stnn of the same two forces. Precisely similar 

 cases of centrifugal movement have been observed, and they 

 furnish another decisive proof of nuclear attraction. 



A capital illustration is furnished in the e^^ of Toxopneiistes 

 lividiis. After saying that the male pronucleus leaves the 



5. Oscar Hertwig. Beitrage z. Kenntniss d. Bildung, Befruchtung u. Theilung 

 d. thier. Eies. Morphol. Jahrb., I., 1876, p. 3S1. 



6. Hermann Fol. Recherches sur la Fecondation et le Commencement de 

 I'Henogenie, 1879, p. 259. 



7. Emil Selenka. Befruchtung des Eies von Toxopneustes variegatus. Leipzig, 

 1878. 



8. Edward L. Mark. Maturation, Fecundation, and Segmentation of Limax 

 campestris. Bull. Mus, Cotnp. Zool , VI., 1881, p. 222. 



