CYTOLOGICAL METHODS

So-called " indifferent" liquids must not be believed to be
without action on nuclei. Iodised serum, salt-solution, serum,
aqueous humour, lymph, better deserve the name of weak
h bardening agents.
Osmic acid (I th to 2 per cent.) preserves the form of the
entire cell, but swells the nuclei and rounds off nucleoli. It
renders the nuclear reticulum    undiscernible. Picric acid,
either concentrated or dilute, and chromic acid, 01 to 0-5 per
cent. are the best fixing agents for nuclei. Stronger chromic-
acid solutions cause shrinking. Neither of these reagents is
harmless as regards the nuclei of red blood-corpuscles. The
salts of picric acid (potash-, soda-, and baryta-salts) are most
harmful. Weak acetic, hydrochloric, or nitric acid combined
with clearing in glycerin, and staining, may be useful for
bringing out reticula and nucleoli. Chloride of gold pre-
serves the forms well, but generally leaves the nuclear struc-
0s  tures unstained. Nitrate of silver is hopelessly uncontrollable
tat.  in its action. Alcohol has much the effect of chromic acid,
but often causes a much greater shrinking of the nuclei.
Bichromate of potash and chromate of ammonia bring out
very sharply the appearance of a reticulum, but these appear-
ances cannot be accepted as true (1. c., p. 334 ff.). Karyo-
kinetic figures are quite spoiled by them.
The best results were obtained with chromic-acid prepara-
tions well washed out and treated for some days with absolute
alcohol before staining with anilin colours (safranin, rose de
naphthalin, &c.). Almost equally good results were obtained
I by means of huematoxylin, which should be " of middle
strength," and allowed to act for at most half an hour, in thc
case of chromic-acid preparations,; whilst in the case of
I picric-acid preparations the humatoxylin should be "as weak
as possible " and be allowed to act for a long time. Mount
jit  in glycerin or in balsam; water is a still better medium for
study of the preparations on account of the stronger images
it affords.  Picro-carmine applied to fresh objects causes
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