192 Macfarlane—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 some 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 isa 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 Sfervogyra, 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. 
