192 Macfarlmie — Current Problems 



when the halves retreat to opposite poles of the nuclear 

 spindle. The growing importance attached to the nucleolus 

 as a result of recent research, tends to confirm the opinion 

 that it may yet be regarded as the most specialized portion 

 of the cell. The recent paper by Dixon contains many good 

 arguments in favor of such a position. It is frequently 

 objected that the nucleolus is a homogeneous mass of 

 formless character, but though its extreme density often 

 suggests this, detailed study reveals minute structural dis- 

 similarities. Further, I have frequently observed during the 

 earlier prophase stages of many cells that the aggregating 

 chromosome substance dips into and fuses with the nucleolar 

 substance, and that both contain elements which stain with 

 equal intensity. This is to som.e degree confirmed by Farmer 

 and Sargant. 



While only one nucleolus seems typically to exist in 

 embryonic cells during the prophase stage of division I have 

 so often observed two, at some distance apart, that their divi- 

 sion, as inaugurating division of the nucleus, is strongly sug- 

 gested. Whether during the metaphase in plants other than 

 Spirogyra, there is a sac-like remnant of the nucleolus left, 

 which divides and in time receives the material that has been 

 distributed along the chromosome, is an unsettled question. 

 In the early anaphase, however, one can generally see deeply 

 staining daughter nucleoli within the nuclei. 



Since the nucleolus behaves chemically and to stains like a 

 nucleo-albumin, since this is more complex than the nucleic 

 acid of chromatin substance, since the nucleolus is the main 

 center of division activity in Spirogyra, and since in many 

 embryonic cells it forms the largest mass of stainable material, 

 and is converted into the bulk of the chromosomes, its prime 

 importance should be conceded. The thought seems to be 

 behind much of our current cytological literature, that it 

 more than the nuclear substance dissolves or loses its identity. 



