236 WHITMAN. ' [Vol. I. 



ccntripete, change, lorsqiie le pronucleus femelle n'occnpe pas le 

 centre de Vceuf, pour se rapprocher de ce dernier noyau. Enfin 

 le pronucleus femelle, jusqu'alors immobile se met en mouvement 

 au moment ou il est attcint par Ics rayojis de Vaster male, ct la 

 reunion des deux noyaux s'opere promptement." 



The same fact is brought out most clearly by the researches 

 of Wilhelm Roux (4) on the frog's ovum. Roux's observations 

 are all the more conclusive, as he has made a special study of 

 the movements of the pronuclei, with a view to determining the 

 precise axial relations of the paths described. The spermatic 

 body takes at first a centripetal direction, penetrating to a 

 depth of .29-. 3 5 mm., and thus describing the ''penetration- 

 path.'' By a more or less abrupt curve the path then becomes 

 directed towards the female pronucleus, — becomes nucleopetal, 

 — and is thenceforth called the " copulation-path y The angle 

 formed by the first and second parts of the course varies 

 from 90'^ to l8o'^, according to the distance of the point of 

 penetration from the pole. The first part of the course is ac- 

 complished through a simple " movement of penetration ; " the 

 second, under the influence of nuclear attraction. Roux does 

 not undertake to assign any reason for the change from a cen- 

 tripetal to a nucleopetal direction, but alludes to the possibility 

 that the nucleopetal course sets in when the transformation of 

 the spermatozoon into the male pronucleus becomes complete. 

 Centripetal attraction, as we have seen, may, under certain 

 conditions, act entirely alone upon one or both pronuclei ; but 

 the attraction between the pronuclei must act, under all normal 

 conditions at least, concomitantly with centripetal attraction. I 

 am aware that the language often used in describing the move- 

 ments of the pronuclei is not in full accord with the last state- 

 ment. Both Hertwig and Fol seem to be very strongly 

 impressed with the idea that the male pronucleus must, under 

 all circumstances, move more rapidly than the female pronu- 

 cleus ; and both declare that the path t-aken by the pronuclei, 

 as they advance to meet each other, is represented by a single 

 straight line, instead of two straight or two curved lines meet- 

 ing at an angle, as here maintained. In one of his latest papers 

 Hertwig (11, p. 281) expresses himself as follows: " Wie nun 



11. Oscar Hertwig. Das Problem der Befruchtung und der Isotropic des 

 Eies. Jenaische Zeitschrifi, XVIII., 1885. 



