114 Transactions of the Society. 
Introduction. 
Since the discovery of the nucleolus in 1781 by Fontana, a great 
number of papers have appeared dealing with its structure and 
functions in both animal and plant cells. Besides various peculiari- 
ties in behaviour within the nuclear membrane, many observers 
have described the extrusion of portions of the nucleolus into 
the cytoplasm during gametogenesis and certain other conditions of 
metabolism. 
There appears to have been much confusion between nucleolar 
and chromatin extrusions. The description of particles of chrom- 
atin being thrown out of the nucleus seemed altogether at variance 
with the theory of the continuity of the chromosomes throughout 
the germ- cell cycle ; and the fact that the latter theory was so 
strongly substantiated by cytologicai studies and breeding experi- 
ments led some biologists to doubt whether any “ solid ” matter 
was normally extruded from the nucleus into the cytoplasm. 
The nucleolus therefore came to be looked upon as an accumu- 
lation of waste products of the metabolism of the chromosomes, 
which was eliminated from the nucleus during cell-division. 
Recently additional interest has been added to the study of the 
nucleolus by several important discoveries. 
Carleton (5)* has shown that during cell-division in certain vertebrates, 
although the nucleolus appears to disintegrate, an argentophile core remains 
intact and passes in the form of granules into each of the newly -formed 
nuclei, in which a new nucleolus forms around them. Then both Gatenby 
and Buchner established the fact that nucleolar extrusions in some insects 
give rise to secondary nuclei (34 ) , while more recently Gatenby has discovered 
that in the marine worm, Saccocirrus , nucleolar extrusions give rise to the 
mass of reserve food of the egg (13). 
These researches, carried out by the best modern techniques, 
and in correlation with a study of the cytoplasmic organs, have 
substantially confirmed the observation of earlier workers, and 
overcome the criticism of those who had maintained that the 
nucleolus is nothing more than the accumulated waste products of 
the nucleus. 
The present paper is the outcome of an earlier one published in 
this Journal (34). In working out the behaviour of the cytoplasmic 
organs during the oogenesis of Patella , a complicated case of 
nucleolar behaviour was met with. The nucleolus was seen to 
extrude fragments of an oxyphil staining substance throughout 
oogenesis. Lack of suitable material prevented the behaviour of 
the nucleolus being followed beyond maturation, so that at the 
suggestion of Professor J. Bronte Gatenby, of Dublin University, I 
carried out this investigation on the aquatic Mollusc, Limnsea ; 
and in the present paper is described for the first time the 
behaviour of the nucleolus throughout the life cycle of an organism. 
* The figures in brackets refer to the bibliography at the end of the paper. 
