218 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
great service in sectioning. When this was desired, the egg, previously 
studied as a whole object, was returned to xylol. The transfer was accom- 
plished by placing the slide on which it was mounted in a shallow por- 
celain dish containing a little xylol. This soon dissolved away the 
balsam, and left the egg free and clearly visible against the white back- 
ground. The egg was next removed to a shallow watch-glass with a 
perfectly flat bottom, which was previously smeared with a thin layer 
of glycerine. Any superlluous xylol was removed from about the egg 
with filter paper, and à small amount of melted paraffine poured over it, 
enough to fill the watch-glass to a depth of 3 to 5 mm. ‘The whole was 
then set over the paraffine bath for fifteen or twenty minutes, when it 
was placed floating on a dish of water to cool. This being accom- 
plished, the paraffine block was removed from the watch-glass, and the 
egg, which of course had settled to the bottom and lay with its long axis 
parallel to the surface of the block, was oriented under the compound 
microscope in any manner desired. "The thinness of the block generally 
allowed plenty of light to pass through it for this purpose, and it was 
usually not difficult, owing to the shape of the embryo, to determine its 
axes. Sections were usually cut 62 y in thickness. 
The staining which was found most advantageous for the study of the 
egg as a whole object was altogether too faint for sections. These were 
accordingly given a further staining after fixation to the slide. Ehrlich's 
heematoxylin was employed, diluted one half with water. After immer- 
sion in the stain for from twenty minutes to an hour, the sections were 
washed in water to remove the superfluous stain, then to decolorize were 
placed in 35% alcohol containing 0.1% hydrochloric acid. Here they were 
allowed to remain until quite pale in color, usually for about five min- 
utes. They were then rinsed in 35% alcohol and held for an instant over 
the unstoppered mouth of an ammonia bottle, a treatment which gave 
the hematoxylin remaining in the sections a deep blue color, and in- 
sured the permanency of the stain. The sections were then passed 
through the grades of alcohol, cleared in xylol, and mounted in balsam. 
This process, when properly conducted, resulted in a beautiful and sharply 
differential double stain. The nuclei retained the light rose tint given 
them by the carminate of lithium, for the superadded hematoxylin stain 
had been entirely removed from them, except in tho chromatio elements, 
which possessed a deep black color. Cell boundaries, attraction spheres, 
and other cytoplasmic structures, were clearly brought out, and the fun- 
daments of various organs, as, for example, chorda, mesoderm, and defini- 
tive endoderm, were distinguished one from another with great sharpness 
