TRANSACTIONS OF SECTION K. 563 
4 coagtilated pulp not visibly granular, or, which may be considered equally pro- 
bable, by an enveloping membrane. Although Kobert Brown was the first to 
recognise the importance of the nucleus, and to give it a name, it had been seen 
by previous observers, and he himself mentions that he had met with indications 
of its presence in the works of Meyen and Purkinje, chiefly in some figures of 
the epidermis; in a memoir by Brogniart on the structure of leaves, and that 
Mr. Bauer had particularly noticed it in the cells of the stigma of Bletéa tanker- 
vilie, but had associated it with the impregnation by pollen. There are some 
figures by Leeuwenhoek, published in 1719, to which Professor L. C. Miall has 
called my attention, of blood-corpuscles of a fish, human epidermal cells, and the 
connective tissue of a lamb, in which nuclei are shown, and they had been seer 
by Fontana (1781) in epithelial cells, and by Cavolini (1787) in some fishes’ eggs. 
To Schleiden and Schwann the cell was essentially a membranous vesicle 
enclosing a fluid sap and a solid nucleus. They thought that it arose in contact 
with the nucleus as a delicate transparent vesicle which gradually increased in 
size and became filled with the watery sap. As soon as it was completely formed, 
the nucleus, having done its work, was either absorbed or cast off as a ‘useless 
member,’ or in some eases was ‘found enclosed in the cell-wall, in which situation 
it passes through the entire vital process of the cell which it has formed.’ So far 
from being the most important organ of the cell, as we now consider it to be, they 
saw in the nucleus merely a centre of cell formation which is no longer required 
when the cell is formed. It was left for Hugo von Mohl to show that the mucus-like 
contents of the cell which he called protoplasm (1846) is the real living matter in 
which reside those activities which call into play the phenomena of life, and 
that the origin of nuclei by division from a nucleus already existing in the parent 
cell would possibly be found to occur very widely. Von Mohl, Nageli, and 
Hofmeister all appear to have had some idea of the importance of the nucleus in 
cell division. Von Mohl says that the ‘ process is preceded in almost all cases by 
a formation of as many nuclei as there are to be compartments in the mother-cell.’ 
Hofmeister’s description of it is interesting: ‘The membrane of the nucleus 
dissolves, but its substance remains in the midst of the cell; a mass of granular 
mucilage accumulates around it: this parts, without being invested by a mem- 
brane, into two masses, and these afterwards become clothed with membranes’ and 
appear as two daughter-nuclei.’ 
It is, however, mainly to the researches of the last thirty years that we owe 
our knowledge of the many complex cell-activities at work in living organisms, 
and we are still only just on the fringe of the great problems which cytology 
has to solve. Some of the most important of these are the origin and evolution 
of the nucleus, the meaning of the complex mode in which the nucleus divides, the 
origin and nature of the spindle figure and centrosomes, the part played by the 
chromosomes in the transmission of hereditary characteristics, the meaning of the 
phenomena accompanying fertilisation, the significance of the longitudinal division 
of the chromosomes and of their reduction in number in the sexual cells, and the 
evolution of the living substance. The satisfactory solution of these problems 
depends upon a clear understanding of the structure of protoplasm and its various 
differentiations. How far we have succeeded in obtaining this I will endeavour 
to show. 
The Differentiation of Structure in the Cell. 
The essential constituent of a cellis the protoplasm. This is differentiated into 
two constituents, the cytoplasm and the nucleus. It is usually held that this 
differentiation is an essential one, and that these two constituents are present in all 
cells ; but, as we shall see later, there is some evidence that not only are there cells 
with very rudimentary nuclei, but cells in which no trace of a nuclear structure 
can be found at all. 
In addition to this primary differentiation of the cell, secondary differentiations 
occur, resulting in the production of organs such as chloroplasts, chromoplasts 
leucoplasts, pyrenoids, and pigment spots, which have special functions to perform 
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