1883.] Microscopy. 573 
since section-cutting came into vogue as before—comparatively 
little progress has been made in overcoming the difficulties that 
attend its preparation for the microtome. The chie difficulties 
are found in freeing the egg from its gelatinous envelope, and in 
preparing it so as to avoid brittleness. 
The best method that has thus far been proposed for these eggs 
is unquestionably that of O. Hertwig, and I shall therefore give 
it in detail. 
I. In order to facilitate the removal of the gelatinous envelope, 
the eggs are placed in water heated almost to boiling a ts 
for 5-10 minutes. The eggs are thus coagulated and somewhat 
hardened, while the envelope separates a little from the surface 
of the egg and becomes more brittle. The envelope is then cut 
under water with sharp scissors, and the egg shaken out through 
the rupture. With a little experience a single cut suffices to free 
the egg. 
2. By the aid of a glass tube the egg is taken up and trans- 
. ferred to chromic acid (one-half per cent), or to alcohol of seventy, 
eighty, and ninety per cent. Chromic acid renders the egg brit- 
tle, and the more so the longer it acts; therefore the eggs should 
not be allowed to remain in it more than twelve hours. While 
eggs hardened in chromic acid never change their form or be- 
come soft when transferred to water, those hardened in alcohol, 
when placed in water or very dilute alcohol, lose their hardness, 
swell up and often suffer changes in form. 
3. Alcoholic preparations are easily stained; but chromic acid 
Preparations are stained with such difficulty and so imperfectly 
that Hertwig omitted it altogether. 
There is an important difference between alcohol and chromic 
acid in their effect on the pigment of the egg. Chromic acid de- 
stroys the pigment to some extent, and thus obliterates, or at 
least diminishes, the contrast between pigmented and non-pig- 
mented cell-layers. As the distribution of the pigment is of con- 
siderable importance in the study of the germ-lamella, it is well 
to supplement preparations in chromic acid with those in alcohol, 
in which the pigment remains undisturbed. 
Sether, so that the thinnest sections can be obtained without dan- . 
 &r of breaking. 
F As the dorsal and ventral surfaces, and the fore and hind 
ends can be recognized in very early stages, it is important to 
know Precisely how the egg lies in the egg-mass in order to deter- 
