416 A. J. NICHOLSON 
persists until after the oocyte has left the resting stage. 
Frequently, however, the chromatin residue is very difficult 
to find in oocytes in the resting stage. Close examination 
reveals the fact that it is not only very closely applied to the 
surface of the nucleolus, but is situated in a shallow depression 
in the latter and thus does not disturb the spherical contour. 
Nuclei in which this arrangement exists appear on a cursory 
examination to contain one large nucleolus and nothing else 
but nuclear sap. This is the characteristic appearance of the 
oocyte nucleus in the resting stage. 
We have noted that the nucleolus, which is at first vesicular 
and has little affinity for stain, soon becomes vacuolated and 
stains more deeply. As it increases in size the vacuoles increase 
rapidly in number and the affinity for stam becomes more and 
more marked, till, when the resting stage is reached, the 
whole surface is covered with vacuoles and the nucleolus stains 
as deeply as chromatin. Though the nucleolus has so great 
an affinity for chromatin stains I do not consider that it is 
formed of chromatin. I regard it rather as a composite struc- 
ture, consisting of a plasmosome in which chromatin, or some 
‘similar basiphil substance, is present. This is indicated by the 
fact that when stained with eosin the nucleolus is stained 
_ bright red, whilst the rest of the ovary is hardly perceptibly 
tinted by it, but when stamed with haematoxylin the red 
colour is completely masked. These staiming properties 
are confined to a cortical layer, in which the above-mentioned 
vacuoles lie. This layer surrounds a large central cavity the 
conteuts of which appears to be nuclear sap. This structure 
of the nucleolus is easily seen as it 1s so large, about 0-015 min. 
in diameter, that 1t may be cut into three or four sections with 
ease. Inu such sections the cortical layer appears as a deeply- 
staining ring surrounding a cavity which contains non-staining 
material, in which irregular strands of another substance, 
possibly linin, can be seen (fig. 17). 
Though the nucleus normally contains only one large 
nucleolus this sometimes appears to undergo fragmentation. 
In the more usual cases of this, one or more small ruclecli 
