236 On the Structure of the Bed [""/ouVS, Xfu'Sea 
little or no attention is given to the few seconds, more or less, which 
are consumed in the manipuktion. It is usually either pressed or 
spread out on the glass slip, and often mixed with water or some 
other fluid. But it is possible to place blood-cells under the micro- 
scope for examination so quickly, and with such shght disturbance, 
that they may be satisfactorily examined before the nuclei have 
begun to form. They may then be shown to be absolutely struc- 
tureless throughout; and, moreover, as the examination is con- 
tinued, the gradual formation of the nuclei can be traced. The 
chief points to be attended to are — to mount a drop of blood as 
quickly as possible ; to avoid as much as possible any exposure to 
air ; to avoid as much as practicable contact of any foreign substance 
with the drop, or any disturbance of it. 
After many trials of various plans, I find that the following will 
often succeed sufficiently well. Having the microscope, and every- 
thing else which is required, conveniently arranged for immediate 
use, an assistant secures the animal which is to furnish the blood 
(say, a frog or a newt), in such a way that the oj)erator may cleanly 
divide some superficial vessel, as the femoral or humeral artery. 
He then instantly touches the drop of blood wdiich exudes with the 
under-surface of the glass, which is to be used as the cover, imme- 
diately places this very lightly upon the slide, and has the whole 
under the microscope with the least possible delay. Thus for several 
seconds the blood-cells may be seen without any trace of nuclei ; 
then, as the observation is continued, these gradually, but at first 
very faintly, appear, and the study of their formation affords strong 
proof of their absence from the living cells. 
The " nucleus " first appears as an indistinct shadowy substance, 
usually, but not always, about the centre of the cell. The outline 
of it can hardly for some seconds be defined, but it gradually grows 
more distinct. Often some small portion of the edge appears clear 
before the rest. At the same time the nucleus is seen to be paler 
than the surrounding substance. Synchronously with this change 
— and this is noteworthy — the outline of the corpuscle (the " cell- 
wall ") becomes broader and darker. What was at first a mere 
edge of homogeneous substance, becomes at length a dark border 
sharply defined from the coloured matter within. Thus a corpuscle, 
at first absolutely structureless, homogeneous throughout, is seen 
gradually to be resolved into central substance or nucleus, external 
layer or cell- wall, and an intermediate coloured, though very trans- 
parent, substance. But — and this is significant — these changes are 
not always thus fully carried out. It not seldom happens that 
the nucleus does not appear as a central, well-defined, regularly 
oval mass. Sometimes it never forms so as to be clearly traced in 
outline, but remains as an irregular, shapeless mass, in its greater 
portion very obscure. Sometimes only a small part, if any, of an 
