30 THE AMERICAN MONTHLY [February, 
variety of methods and experimented in various ways upon both staining and 
imbedding. The best methods are extremely tedious, but if perfect success 
results tediousness is by no means a fatal objection. Of all tissues embryonic 
tissues require the most careful handling. We have had good success by 
several methods, but we can recommend either picric acid or corrosive sub- 
limate as the best reagents for hardening. 
HARDENING. 
I. Corrosive sublimate is to be used in a saturated solution, the embryo 
to be left in about one-half hour, then transferred to distilled water, where it 
may remain one-half hour or a little longer if the chick be of over two days’ 
incubation. The purpose of the water is to thoroughly remove the corrosive 
sublimate, which has served its purpose in the rapid hardening of the speci- 
men. ‘The corrosive sublimate solution is best made by heating water con- 
taining the salt to boiling, dissolving up as much of the salt as possible. Then 
the solution should stand till cool. A great deal of the salt will be crystallized 
out of the solution, but a perfectly saturated solution will result. This is to 
be used at the temperature of the liquid standing in the room. In hardening 
it is always wise to immerse the specimen in at least ten times its bulk of 
the reagent. 
Il. Pécrzc ac¢d solution is prepared in a variety of ways. We will give 
one which is a very good one. Atsome future time we project a full review of 
the methods of hardening tissues. Prepare a saturated aqueous solution of 
the picric acid crystals. Filter the solution and add to it 2% of strong nitric 
acid. A heavy precipitate will fall; filter the mixture, which is popularly 
known as Meyer's picro-nitric. It will remain unchanged an indefinite 
length of time ready for instant use. In practice this is generally diluted by 
the addition of three parts of distilled water to one of the picro-nitric. A speci- 
men should be left in picro-nitric fluid about three hours. If the chick is 
old enough so that it is bony anywhere, the picro-nitric should be used full 
strength and the specimen remain in it 6 hours. By this time decalcification 
will have taken place. 
From the water after corrosive, or from the picro-nitric, if that be used, the 
chick is to be transferred to 30% alcohol $ hour, then to 50% alcohol $ hour, 
then to 70% alcohol. The corrosive specimen, after a couple of changes of 
the 70% alcohol, will be ready for staining. The picric specimen must be 
kept in 70% alcohol, changed every 24 hours, till the alcohol is no longer 
colored by picric washed out of the specimen. 
STAINING. 
It is best to color the specimen in borax-carmine, or Kleinenberg’s hzma- 
toxylin is also very satisfactory, before cutting the sections. Picro-carmine 
would be better with picric acid chicks, if one could be sure of possessing 
the reagent in the perfect condition, but with borax-carmine there is not the 
slightest difficulty. The chick should be left 24 hours in borax-carmine, then, 
after a short wash in acidulated 70% alcohol, it should be transferred to 70% 
alcohol. : 
IMBEDDING. 
The specimen thus hardened and stained is to be imbedded as follows :— 
1. Transfer to liberal amount of g0% alcohol 6-24 hours, according to size. 
2. Transfer to absolute alcohol 6-24 hours. 3. From the absolute pass into 
spirits of turpentine and leave here till no more alcohol can be removed 
by the turpentine, when saturation is complete, as shown by the absence 
of current visible about the specimen, as well as by the translucent ‘ cleared ’ 
appearance, 6-24 hours. 4. From turpentine to a saturated solution of 
paraffine in turpentine, 6-24 hours. 5. From paraffine and turpentine to 
