ta.n                        CHAPTER         II.
ferable                FIXING AGENTS.
rQ1o is
a      ti.   4. By the fixing of tissues is meant the rapid killing of
4. and   their elements, together with such a degree of hardening as
pood   suffices to fix them  in their natural forms, and enable them
bith   without distortion or other injury to undergo such further
d the  treatment with staining agents or other chemical reagents,
)hi.   such hardening or clarifying processes, as may be desirable.
D8ls it  I do not, as some writers appear to do, restrict the meaning
Qeled  of the term to the instantaneous killing and hardening of
action  protoplasm' alone, but include also under it a preliminary
LoulY  hardening of the formed tissue-elements, fibres, tubes, mem-
on of  branes, and so forth. An example or two will make evident
both the meaning of the term and the importance of the
process in all researches into delicate structures. In the
course of their beautiful inquiry into the " Development and
Retrogression of the Fat-cell " (vide 'Journ. Roy. Mic. Soc.,'
vol. xi, p. 353) the Hoggans investigated the relations subsist-
ing between the wandering cells found on serous membranes
U  and the fat-cells also found there. They chose as an object of
study the growing broad ligament of pregnant rats and mice.
They drenched the animals with chloroform, with the idea of
anoesthetising the individual cells of its tissues, they stretched
the portion of membrane to be examined over vulcanite rings,
and treated it with solution of nitrate of silver, in order not
only to fix " the various cells forming the membrane in the
condition or shape they possessed during life," but also in

11

FIXING AGENTS