TRANSACTIONS OF SECTION K. 573 
the cell wall by delicate strands of cytoplasm, also full of moving granules. The 
first indication of division is shown in the accumulation of cytoplasm around the 
nucleus and the gradual disappearance of the nucleolus; at the same time the 
originally somewhat oval or spindle-shaped nucleus becomes rectangular in shape 
whilst the nucleolus disappears entirely. Shortly after this the nuclear plate 
appears as a pale refringent structure, just like the original nucleolus, but of a 
different shape. As the nuclear plate is formed the homogeneous substance in 
the cavity of the nucleus presents a finely striated structure on each side of the 
nuclear plate and at right angles to it extending to both ends of the nucleus. 
The nuclear membrane becomes indistinct ; the nucleus elongates and an accumu- 
lation of granular cytoplasm begins at its two ends or poles. 
The nuclear plate then divides, and the two halves begin to move apart; the_ 
accumulation of cytoplasm at the poles becomes more prominent and appears to be 
closely connected to spindle fibres attached to the two halves of the nuclear plate. 
The spindle fibres gradually become shorter, but I could not detect any appreciable 
thickening of them, The impression conveyed to my mind was that the substance 
of them was being absorbed into the mass of cytoplasm at the poles, which was 
now rapidly increasing in size and forming very distinct polar caps. Shortly after 
the two halves of the nuclear plate had begun to move towards the poles the con- 
necting threads became visible between them, and in the later stages of division 
became very prominent, less numerous, and thicker, probably by fusion, but I 
could not be certain of this. When the flat halyes of the nuclear plate reach the 
poles they contract into somewhat irregular oval-shaped masses, the daughter- 
nuclei, These become more definite in outline; a nucleolus appears in each, a 
nuclear membrane is then formed, and the daughter-nuclei are thus completed. 
This does not of course give us a complete picture of all the details visible in 
fixed and stained specimens, but sufficient to show that the more important features 
of mitosis have a very real existence, and are not due to reagents. 
The Structure of the Chloroplast. 
Tn view of its extreme importance in the function of assimilation a knowledge 
of the structure of the chloroplast is important. Owing to its small size a satis 
factory demonstration of its finer structure is very difficult. That it consists of a 
colourless ground substance, in which the chlorophyll is embedded, is clear; but how 
these two substances are united and the relations between them structurally are 
not known. Pringsheim concluded that the ground substance of the chloroplast 
is a sponge-like network with the oil-like solution of chlorophyll in its meshes. 
Schmitz thought that the fine granular appearance of the chloroplast was due 
to a fine net-like structure in which the chlorophyll was diffused. Fromman also 
describes it as a green granular network. Schwartz, on the other hand, describes 
it as composed of a ground substance containing a number of green fibrillz side b 
side, which are coloured green throughout, but show also an accumulation of the 
green colouring matter in the form of granules along these threads, 
Meyer thought it was composed of a homogeneous ground substance with 
various-sized granules of the green substance embedded in it. To these granules 
he gave the name of ‘grana.’’ Schimper stated that it was composed of a colour- 
less stroma containing numerous vacuoles filled with the green semi-fluid chloro- 
phyl, identical with the ‘grana’ of Meyer. 
Some observers consider that the chloroplast is surrounded by a distinct mem- 
brane ; whilst others consider that the substance of the chloroplast is directly 
connected by colourless strands to the cytoplasm, 
According to some observations which I have recently made, the chloroplast, 
when examined under high powers in the living condition, appears to be filled 
with a mass of green granules with a colourless substance between them. But 
in certain cases a distinct fibrillar arrangement of the chlorophyll is observed, 
This is very easily seen in the chloroplasts of Exglena, both in the living con- 
dition, and, more easily, when the cells are burst and the chlorophyll grains are 
