EMBRYOLOGICAL METHODS

days to harden in strong alcohol. Sections are mounted in
glycerin without removing the attached portions of the jelly.
ii 494. Mounting Chick Embryos Whole.'-This is easily done
I by excising a blastoderm under salt-solution, floating it on to
a slide, and there fixing, staining, dehydrating, and finally
i mounting it. It is, however, necessary to take special pre-
cautions to prevent the borders of the blastoderm       from
curling-up during the hardening process. This may be pre-
vented by means of the method of demi-desiccation.       The
fresh blastoderm is allowed to begin to dry until its edges
adhere to the slide; it may then safely be brought into the
fixing solution.
Fix with 0-1 to 0-5 per cent. chromic acid (twenty-four
hours), and wash-out with water followed by alcohol, or with
osmic acid. In this case, after washing out, the preparation
should be treated for some hours with some reagent capable
(of preventing the osmium from after-blackening (Miiller's
solution, chromic acid 0-5 per cent., Merkel's solution).
Stain as desired, dehydrate, clear with clove oil, and mount
i in balsam.
495. Sections of Chick Embryos.2-For embryos of thirty-
six to forty-eight hours Foster and Balfour recommend the
following methods:
The embryo is to be removed under the surface of salt-
solution (0-75 per cent.) kept at a temperature of 380 (the
shell being broken into above the air-chamber, and removed
piece by piece with forceps until the embryo is exposed).
The embryo should then be floated on to a slide, fixed in
place there by demi-desiccation as recommended above, and
brought into the fixing solution. Foster and Balfour recom-
mend chromic acid of 0-1 per cent. for twenty-four hours,
followed by 0-3 per cent. for twenty-four more; after which
70 per cent. alcohol one day, and 90 per cent. alcohol two
1 Foster and Balfour, ' El6ments d'Embryologie,' Appendice.
2 L. c.

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