782 Kemp .— Oil the Question of the Occurrence of 
time. It must be clearly borne in mind that although the moments of 
fixation are of necessity chosen arbitrarily they do not represent sharply 
defined stages in the action of the drug, or in the reaction of the tissue to 
the latter. That action is essentially a continuous one, and its effects must 
be traced as such through the periods subsequent to the experiment. 
1 and 2. After 1 hr. in 0*75 % chloral hydrate and 1 hour s washing. 
The two first fixations present practically the same features, and may 
be described together. The poison is seen to have had a marked effect 
upon the entire tissue, which is diffusely stained and lacking in sharply 
defined details. This appearance is particularly noticeable in the periblem 
and outermost cell layers, the denser meristem and rows of cells within the 
endodermis being rather more normal in appearance. The cell walls are 
seen to be slightly swollen, and the cytoplasm, especially that of the external 
layers, is vacuolated and thickly speckled over with small fragments of 
chromatin (PI. LXVI, Fig. 1). The number of nuclei in division is very small; 
the majority being in complete rest, finely granular in appearance, with ill- 
defined membranes and taking the stain feebly (Fig. 1). The only constituent, 
indeed, of the resting nuclei which is well stained is the nucleolus. The 
latter is surrounded by a clear space and often divided into two or more 
bodies. Very few nuclei in spireme are visible, and these almost all in the 
internal cell rows. Their coiled threads show conspicuous longitudinal 
division (Fig. 1). The effect of the poison is seen most strikingly, however, 
in such nuclei as are in division. These without exception show a marked 
alteration in character ; there is a complete absence of achromatic fibres, 
and the chromosomes, which have a pronounced longitudinal split, are either 
crowded together at the centre of the cell, or scattered irregularly across it, 
and at the equatorial plate stage their split halves show no definite arrange¬ 
ment on a plate (Figs. 4 and 5). In the diasters the two groups of chromo¬ 
somes, instead of going to the poles, form two irregular masses at a little 
distance from each other, and in many cases still partly joined together by 
strands of chromatin. At the telophase, these irregular imperfectly divided 
masses of chromosomes are becoming vacuolated, preparatory to going into 
rest in that position, or in those cases where division has been complete, the 
two masses form two daughter nuclei close together, and flattened on their 
contiguous sides. In the majority of such binuclear cells there is no sign 
of a cell-plate between the two nuclei, or at most only a slightly denser 
plasma (Fig. 1) ; a few, however, have a fragment of plate at the side or 
centre of the cell. Other cells again contain, instead of two separate nuclei, 
one long lobed nucleus, of the kind known as a ‘ bridge ’ nucleus, with 
an unfinished cell-plate in the curve of the bridge. It is evident that these 
binuclear and ‘ bridge ’ nuclear cells result from the various degrees of 
imperfect division described above. The chromatin of the division figures- 
