THE MICROSCOPE IN ANIMAL HISTOLOGY. 205 



bral vesicles, the folds of the somatopleure and splanch- 

 nopleure, the provertebne, etc. 



To prepare sections of the embryo, it must be first hard- 

 ened by placing the slide containing it in a solution of 1 

 per cent, chromic acid for twenty-four hours. From this 

 it should be removed to one of 3 per cent, for twenty -four 

 hours more ; then for a similar time in alcohol of 70 per 

 cent., then in alcohol of 90 per cent., and lastly in abso- 

 lute alcohol, where it may remain till required for section. 

 Sometimes picric or osmic acid is used for hardening. 

 The embryo may be stained by placing it in Beale's car- 

 mine fluid for twenty-four hours, and then replacing it in 

 absolute alcohol for a day before it is cut. It may also 

 be stained with hsematoxylin if preferred. The specimen 

 may be imbedded in paraffin, wax, and oil, or a mixture 

 of four parts of spermaceti to one part of cocoa butter or 

 castor oil. If there are cavities in the object, it is best 

 to saturate it first with oil of bergamot. A little melted 

 spermaceti mixture is poured on the bottom of a small 

 paper box, and when solid the embryo is placed flat on 

 it, the superfluous oil removed as far as possible, and the 

 warm mixture poured on. Bubbles can be removed with 

 a hot needle. A mark should be made of the exact posi- 

 tion of the embryo. Sections may be cut with the sec- 

 tion-cutter or a sharp razor, and if the spermaceti mix- 

 ture is used, the razor should be moistened with olive oil. 

 The sections should be floated from the razor to the slide, 

 and treated with a mixture of four parts turpentine and 

 one of creasote. They may then be mounted in balsam 

 or dammar varnish. 



The most instructive transverse sections of an early 

 embryo will be through the optic vesicles, the hind brain, 

 the middle of the heart, the point of divergence of the 

 splanchnopleure folds, the dorsal region, and a point where 

 the medullary canal is still open. For the unincubated 

 blastoderm only one section, through the centre, is re- 



