125 
The Morphology and Physiology of the Nucleolus. 
cases where amitosis has been observed in living cells, it has not 
generally taken place according to Remak’s scheme. 
Other theories of nucleolar function have attributed to this 
organ, functions in connexion with the achromatic structures 
which play such an important part during karyokinesis. One of 
the most important of these theories is that of Strasburger (So), 
who suggested that the nucleolus in certain plants was a mass of 
reserve kinoplasm which indirectly formed the achromatic figure. 
The above outline of theories of the function of the nucleolus 
is not intended in any way to be inclusive, but is given in order 
to show the diverse views that have been entertained by those 
who have worked on the subject. Probably the nucleolus does 
not serve the same purpose in all cells, or may be it performs 
normally a variety of functions, so that there may be a certain 
element of truth even in the most divergent opinions that have 
been held as to its function. 
Preliminary Investigations with Fresh-water 
Molluscs. 
At the commencement of this investigation several different 
species of our common fresh-water molluscs were collected and 
kept in aquaria. 
In the early summer when ovulation had commenced, gonads 
were dissected out from a specimen of each species, and prepara- 
tions were made from them and examined microscopically. The 
extrusion of plasmosome substance from-4he nucleus into the 
cytoplasm was observed in Planorbis am in Limnsea stagnalis and 
L. auricularia, which were specially studied. Of these species 
L. stagnalis seemed to be the most suitable for detailed study, first 
because the eggs were found to be the easiest to manipulate, and 
secondly because yolk formation in the oocyte and the behaviour 
of the Golgi apparatus and mitochondria during embryogeny have 
already been worked out by Gatenby (11). 
Ontogeny of Limn^a stagnalis. 
Limnsea stagnalis is one of our commonest aquatic pulmonate 
molluscs, the size it attains being determined within limits by the 
size of the pond in which it lives, the biggest specimens occupying 
the largest ponds. It is hermaphrodite, and within its gonads 
both male and female germ cells are to be found in all stages of 
development. 
Apparently it is self sterile, the eggs of one individual only 
being fertilized by the sperms of another. 
During the late spring and early summer, numerous eggs are 
laid in the form of clusters, their number varying from about 
fifteen to over a hundred. Fertilization occurs after laying. Each 
