Franklin P. Mall 435 
We have gradually learned that it is best to clear specimens which 
have been well shrivelled in alcohol in a 1% solution of KOH for a few 
hours and not in the strong solution recommended by Schultze. With 
the weaker solution the tissues, of the smaller embryos especially, remain 
firm, and, in the end, the specimen is perfectly transparent with all the 
bones held in place. After the treatment with potash the embryo is 
placed in the following solution for days, or even for months: 
IPCs M tet crt, cau der MEL Cae NES, lee at uet Ses ed note (ttc ar/oh tiles oie Voha tt 79 
AG tee MN tracy enlace tees ce ett a epee een ne nk chad Aleun tape 20 
MNO TRS HID Cas eee ee eee ote erode oad wee ch evehs Bre Saeite hayes: Mietea, Samer 1 
From time to time the embryo may be returned to a 3% solution of 
potash for a number of hours in order to hasten the process. The action 
of the glycerine has been to make the tissue more resistant, and for this 
reason the strength of the potash is increased. In case there is much 
blood pigment in the embryo or it is otherwise colored through age, this 
may be removed by placing the specimen for a number of days in the 
strongest ammonia to which a little potash has been added, as recom- 
mended by Hill.° In case the embryo is stained with alum cochineal be- 
fore it is cleared the bones alone are colored red, and for the study of 
their finer connections this is often an advantage. 
Many of the embryos received recently have been preserved in formalin 
and for a long time it was practically impossible to clear them in the 
ordinary way. Finally, in desperation, I placed such an embryo in a 10% 
solution of potash and to my astonishment I found that it began to clear 
at the end of a month. By further treatment it was found that such em- 
bryos could be cleared perfectly well, and in case the bones are not decal- 
cified by the formalin the very best specimens are obtained. In them the 
tendons and other white fibrous tissue are rendered tougher than in the 
specimens treated with alcohol alone. Finally when the embryos are well 
cleared they are gradually transferred to stronger and stronger glycerine 
until all the water is removed from them. 
Before clearing the embryo it is well to cut it through the sagittal plane 
into two equal parts, for by this treatment the bones are all brought into 
view in the finished specimen. Later these halves may be fixed to glass 
slides with gelatin, as recommended by Bardeen. The specimen is to be 
taken from pure glycerine and wiped gently and then placed upon a 
glass plate which is covered with melted gelatin. As soon as the gelatin 
is cool this slide with the embryo attached is returned to pure glycerine 
8’ Hill, Johns Hopkins Hospital Bulletin, 1906. 
