162 EMBRYOLOGY. 



for further treatment in one or two hours. It may be stained 

 by steeping it for twenty-four hours in very dilute carmine 

 (see Chapter VII.), and it is then washed in weakly acidulated 

 water. The object is now placed in absolute alcohol for from 

 half an hour to an hour. After this, it is embedded in the 

 following manner: A layer of the mass used for embedding 

 (wax and oil) is poured upon aflat piece of glass, wood, or cork, 

 or into a little box, and is allowed to harden ; the object, after 

 its surface has been earefull}" dried, is placed in the desired 

 position upon this mass, and a further layer is poured around 

 and over it, which must be warm, but not too hot. When the 

 mass is thoroughly solidified, sections are made as follows: 

 The razor is moistened, by means of a small brush, with oil of 

 cloves or with turpentine, and a section made, which is floated 

 off from the razor to an object-glass with oil of cloves. When 

 the section is thoroughly transparent, a process which occupies 

 a few seconds, or at most minutes, if the object has been long 

 enough in absolute alcohol before embedding, the excess of oil 

 of cloves is to be carefully soaked up with strips of filter-paper. 

 A window is cut out of fine tissue paper, and applied to the 

 preparation in such a way as to afford protection from the 

 pressure of the cover-glass. A drop of Dammar varnish is 

 allowed to fall upon the preparation thus inclosed by the paper, 

 and the whole is covered. The eggs having been placed in 

 one-tenth per cent, solution of chromic acid until the gelatinous 

 investment is entirely dissolved, the} r are transferred to common 

 alcohol for two or three days and then preserved in glycerin. 

 They may be used even after an interval of months. 



For the study of the cleavage-cavity of Batrachia, sections 

 should be made of the eggs of Bufo, beginning with the stage 

 at which the first furrows are already formed. The egg is 

 taken, by means of a spoon, out of the glycerin, dried with 

 filter-paper, and embedded according to the method above 

 described. The razor in this case is to be moistened with 

 absolute alcohol, and the sections floated on to the object- 

 glass, with the same liquid. The alcohol is removed by filter- 

 paper, and the section moistened with a drop of oil of cloves, 

 after which the process is the same as above. Batrachian 

 eggs require great care and attention, both in making and 

 handling the sections; first, because the ovum is less easily 

 fixed than is the case with the disk-like germ of the trout or 

 chick, and, further, because it is extremely friable, so that 

 sometimes, out often sections, only one will be brought entire 

 under the cover-glass. The first indication of a cavity may 

 be traced shortly after the appearance of the first two furrows. 

 In sections made at this stage, it is seen that the upper two 

 quarters of the germ, that is to say, those furthest removed 

 from the white pole, and which are always smaller than the 



