140 J. T. CUNNTNGHAM. 



The author fiuds that during the enlargement there is a great 

 increase of substance, during diminution a great loss of sub- 

 stance, in the chromosomes. He regards this temporary sub- 

 stance as the somatic plasm, whose function is to govern or 

 cause the growth of the egg and the accumulation of the yolk j 

 when this object is accomplished it merges in the cytoplasm, 

 i.e. becomes ordinary cytoplasm, and the chromatin which is 

 left is that which is concerned in fertilisation and heredity. 



The nucleoli experience in the second period of the develop- 

 ment of the egg a reduction in mass, which proceeds at a rate 

 equal to that of the diminution of the chromosomes. The 

 nucleoli diminish in size, become paler, and then vanish alto- 

 gether, following the chromosomes during the process towards 

 the centre of the vesicle. 



According to B. HoU, in the human ovum the chromosomes 

 disappear altogether, and the nucleolus (here single) takes 

 their place, breaking up into little spheres which become the 

 chromosomes of fertilisation. But Kiickert considers that 

 this is an error. 



The third period in the history of the ovum, according to 

 Ruckert, is the formation of the polar bodies. At the be- 

 ginning of this period the chromatin-figure is surrounded by 

 a remnant of the germinal vesicle without any membrane, and 

 much smaller than the original vesicle. The chromatin forms 

 a dense heap, 6/x in diameter. It seems at first to be homo- 

 geneous, but is really a dense coil formed of the rods of the 

 previous stage. The heap separates again into separate cor- 

 puscles, and these form the equatorial plate of the first direc- 

 tive spindle. 



In a later paper (1892) Riickert has described how he en- 

 deavoured to ascertain the stage at which the doubling, or 

 longitudinal division of the chromosomes took place. He 

 found that they were already double in eggs of ^ to i mm., in 

 which the chromosomes are most indistinct, and, indeed, in 

 ova fixed with sublimate, invisible. They can, however, be 

 traced in the form previously described in eggs fixed with 

 riemming's mixture. He points out that in younger eggs the 



