FIXING AGENTS

cai   whole of the rods and cones will be found perfectly preserved
I    after twenty-four hours' time, and the nerve-fibres will be
a    found not to be varicose. After this preliminary hardening,
a16a  portions of the retina may be treated with water (which
would be ruinous to the structures of a fresh retina), they
the   may even remain in water for days without harm; they may
6rs   be stained, acidified, hardened, imbedded, cut into sections,
tiat and mounted in either aqueous or resinous media without
sIS  suffering.
be For the study of the intimate processes of segmentation of
lOt   ova, and division of cells generally, it is evident that the
Im    employment of fixing agents is a necessity; for it is a princi-
nee,  pal object of such study to seize upon particular moments of
ed    very transient phenomena, to investigate structural arrange-
inot  ments that exist only during a fleeting instant of cell-life,
osE   and that vanish without leaving a trace behind if the cells
I    are allowed to die off slowly. The researches of Fol on the
nog    phenomena attending the impregnation of the ovum, and of
Out   Flemming and Strasburger on the division of cells and nuclei,
61.   could not have been carried out without some means of
tin quickly rendering permanent the transient states it was
1of    desired to study.
A Fixing agents, then, afford a means of preserving a record
a of moments of vital activity that would otherwise be inacces-
0o3   sible to observation. They also afford a means of seizing on
iu transient states of functional activity, and preserving them
en    for leisurely study. Ranvier wished to investigate the rela-
Ite   tions of the different parts of muscle substance in the
d     different stages of contraction and relaxation of the muscle.
)10,   Study of the living muscle would not suffice, as it is impos-
ort    sible to follow with the eye the rapid changes by which a
be muscle passes from the state of contraction to that of relaxa-
t.   tion or the reverse. By the injection of a drop of osmic acid,
of     muscular tissue is instantly fixed in the form it has when the
he  acid comes into contact with it, and specimens of muscular

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