A Cytological Study of Artificial Parthenogenesis etc. 157 



be ignored in comparing the processes of natural and chemical 

 fertilization. 



In both cases the first effect of fertilization is to start a cytolytic 

 process resulting in the formation of a membrane. In naturally 

 fertilized eggs the sperm nucleus in its passage through the cytoplasm 

 is accompanied by a radiation which comes in contact with the egg 

 nucleus and there spreads out on one pole of it. The fusion of the 

 two nuclei which now takes place is followed by a period of nuclear 

 growth during which the radiations almost disappear. Meanwhile 

 the nucleolus exhibits unmistakeable signs of dissolution, as it 

 changes from being a dense spherical body to a faintly staining and 

 often indistinct mass. 



In the chemically fertilized eggs the changes are very similar. 

 The egg nucleus becomes surrounded by a perinuclear zone with 

 faint radiations extending into the cytoplasm. This stage probably 

 corresponds with the radiation that appears near the sperm nu- 

 cleus and forms a clear zone at one end of the cleavage-nucleus 

 after the two germ nuclei have fused. Moreover, just as in the 

 normal eggs, it is succeeded by a period of nuclear growth during 

 which the perinuclear zone with its radiations practically disappear 

 and also the nucleolus shows signs of dissolution. The appearance 

 of this radiation is probably the result of oxidation processes starting 

 from the nucleus, which result in the solution of the yolk granules 

 and is followed by the flowing of these solutions towards the nucleus. 

 They are used up in the synthesis of chromatin,, and hence this 

 process is followed by an increase in the size of the nucleus. With 

 the cessation of nuclear activities preparatory to division cytoplasmic 

 solutions no longer flow towards the nucleus and hence the radia- 

 tions caused by these centripetal currents disappear. From the 

 alteration in its appearance it is probable that the nucleolus also 

 takes some part in the nuclear syntheses. 



In both kinds of eggs the period of nuclear growth is succeeded 

 by an apparent cessation of nuclear activities preparatory to division. 



In naturally fertilized eggs a distinct aster (cleavage aster) 

 appears at one pole of the nucleus, its rays centering in a clear 

 area which represents a diffuse centrosome. This area divides and 

 the two halves move apart until they come to lie at opposite sides 

 of the nucleus and form the poles of a typical amphiaster which is 

 developed in the nuclear region. Meanwhile the chromatin assumes 

 the form of a spireme, which breaks up into 36 chromosomes that 



