No. 2.] THE EGG OF BUFO LENTIGINOSUS. 331 



of cytoplasm. The male pronucleus, on the other hand, is 

 always in close contact with the astrosphere, lying between 

 the two centers after their separation and being surrounded by 

 the same pigment layer enclosing them at the stage of Fig. 50. 



Fick considers the migration of the two pronuclei to be due 

 to amoeboid movements ; but the pseudopodium-like processes 

 which he finds in the pronuclei of Axolotl are not present 

 in the ^.^■g of Bufo. It seems more probable, as Wilson ('96) 

 says, that : " The paths of the germ-nuclei are determined 

 by at least two different factors, one of which is the attrac- 

 tion or other dynamical relation between the nuclei and the 

 cytoplasm, the other an attraction between the nuclei. The 

 former determines the entrance path of the sperm-nucleus, 

 while both factors probably operate in the determination 

 of the copulation path along which it travels to meet the 

 egg-nucleus." 



The place of union of the two pronuclei is never the geomet- 

 ric center of the Qgg, but is much nearer the upper pole, appar- 

 ently in the neighborhood of the position occupied by the 

 germinal vesicle just before its dissolution. It is impossible 

 to distinguish between the two pronuclei just before they have 

 fused. They are both about 0.024 mm. in diameter, and in 

 each the reticulum has become very fine and stains but faintly, 

 while the rounded nuclear masses are much more numerous 

 and stain more deeply. About three-quarters of an hour after 

 the egg is fertilized, the two pronuclei fuse completely. The 

 segmentation nucleus is at first decidedly oblong (PI, XXXI, 

 Fig. 56) and shows a pronounced reticulum, in the meshes of 

 which are the rounded nuclear masses. When the segmenta- 

 tion nucleus rounds up (PI. XXXI, Fig. 53), the reticulum 

 begins to disappear and all the nuclear bodies become collected 

 at the nuclear membrane. 



Meanwhile the astrospheres have undergone a considerable 

 transformation. From an irregular meshwork (PI. XXXI, 

 Fig. 51), the fibers of each astrosphere become arranged radi- 

 ally, as in Fig. 56. The central parts of these radiations show 

 no centrosome, and, as in the case of the astrospheres at the 

 poles of the first polar spindle, they appear to be formed only 



