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The Morphology and Physiology of the Nucleolus. 
The early part of the research was carried out in Professor 
J. P. Hill’s laboratory, at University College, London. My thanks 
are due to him for the facilities there offered, and for his interest 
in the work. I have also to thank Professor J. Bronte Gatenby 
for kindly reading through the completed manuscript. 
To Dr. J. A. Murray, Director of the Imperial Cancer Research 
Pund, I am indebted, for examining some of my material and 
assisting me with his advice. 
Previous Work. 
1. “Solid” Extrusions from the Nucleus during Gametogenesis 
and Development . — The nature of the relationship existing between 
the nucleus and the cytoplasm is one of the fundamental problems 
of cytology. Certain writers hold the opinion that the nucleus 
controls and directs the activities of the cytoplasm by the extru- 
sion of small particles of chromatin into it ; such chromatin bodies 
they term “ chromidia.” Schaxel (SO) ascribes to them an all- 
important role in the conversion of the undifferentiated embryonic 
cells into the specialized cells of the somatic tissues. According 
to this writer, in the early stages of the development of the 
fertilized ovum, there is no extrusion of solid substance from the 
nucleus, but this process occurs as soon as differentiation and 
specialization of the cells commence. These conclusions are open 
to a considerable amount of criticism, as other investigators 
working upon the cytoplasmic organs in development, notably 
Duesberg (10) and Gatenby ( 11 ), have observed bodies similar to 
the chromidia of Schaxel, which they have found to be mito- 
chondria. It is therefore extremely doubtful whether there are 
such bodies as the “ chromidia ” of Schaxel, which function in the 
way he describes. 
There does, however, appear to be an extrusion of “ solid ” matter 
from the nucleus of Protozoa (9), but whether the extruded sub- 
stance is chromatin, is difficult to say. Hertwig (3S) describes in 
Actinosphserium an excessive enlargement of the nucleus in com- 
parison with the cytoplasm, under certain conditions of growth, 
which is followed by the extrusion of what he calls chromatin 
into the cytoplasm, where it becomes converted into a brown 
pigment. As I have pointed out in a previous paper (34), it is 
practically impossible on staining tests alone to distinguish 
chromatin, and the extruded nuclear substance may possibly be 
nucleolar in origin. The nature and function of the nucleolus, 
however, is quite unknown, so that it is desirable to bear in mind 
in this connexion that certain observers look upon the nucleolus 
as the centre for the accumulation of chromatin, preparatory to its 
extrusion from the nucleus (3). 
During gametogenesis, numerous cases of extrusions from the 
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