232 WHITMAN. ' [Vol. I. 



point of meeting (-f'), and then along the radius in which this 

 point hes to the centre (^). If, on the other hand, it varies, it 

 may do so in many ways, three of which may be noticed here. 

 Allowing, what seems most probable, that 

 the centripetal attraction steadily dimin- 

 ishes, the zero-point being reached at the 

 centre, we may assume (i) that the nuclear 

 attraction remains constant; or (2) that it 

 increases as the distance between the pro- 

 nuclei diminishes; or (3) that, acting only 

 at relatively short distances, it does not 

 come into play until the centripetal migra- 

 tion is partially completed. In the first 

 two cases the paths of the pronuclei would be represented 

 by curves, with the concave sides towards the pole, with the 

 difference only that the curves would be stronger in the second 

 case than in the first. In the third case the paths would continue 

 straight until nuclear attraction began to act, and then curve 

 towards the pole until the point of junction was reached. In 

 all three cases the course after meeting would be centripetal 

 along the radius in which the point x (Fig. 2) lies. Essentially 

 the same thing would happen in a telolecithal egg, except 

 that the point c, representing the centre of equilibrium rather 

 than the geometric centre, would be nearer the active pole. 



In the movements thus far considered there are only two 

 peculiarities, the occurrence of which could be regarded as 

 conclusive evidence of nuclear attraction ; namely, the curved 

 paths of the pronuclei (Fig. 2 ), and their meeting before reaching 

 the centre. The first of these peculiarities is remarkably well 

 shown in the amphibian egg (i, 2, 3, 4), where the path of the 

 male pronucleus is plainly marked by a streak of pigment con- 

 cave towards the dark pole ; and the second has been repeatedly 



1. Oscar Hertwig. Beitrage zur Kenntniss der Bildung, Befruchtung und 

 Theilung des thierischen Eies. Morph. Jahrb., III., 1877. 



2. Charles Van Bambeke. Recherches sur I'Enibryologie des Batraciens. 

 Bull, de I'Acad. roy. de Belgique, LXL, 1S76. 



3. Id. — Sur les trous vitellins que presentent les ceufs focondes des Amphibicns. 

 Bull, de I'Acad., etc., 2e ser., t. XXX., 1870. 



4. Wilhelm Roux. Beitrage zur Entwickelungsmechanik des Embryo. Arch. 

 f. mik. Anal., XIX., II. 2, PI. x., 1887. 



