20 THE AMPHIOXUS. 



a living state for the purpose of exactly determining the stage. 

 The glass too was marked with a number, because in the case 

 of the embryos which had been subjected to a dark staining 

 and treated with reagents, an exact determination of the staga 

 was not so readily possible as before. At the same time, a 

 record was made in which the stage, as determined by ex- 

 amination of the living object, was accurately marked with 

 reference to the number. In this way I collected in unbroken 

 succession complete series of embrj-os ready prepared to be cut 

 into sections. 



In cutting the sections the main point is that the embryos 

 should be examined and placed in the embedding material 

 accurately with reference to the direction of the sections. The 

 embryo is taken out of absolute alcohol, placed upon a slide, 

 and cleared with a drop of oil of cloves. The oil of cloves is 

 spread over the whole slide, so that the embryo lies there just 

 wetted. Then a few drops of a slightly warmed mixture of 

 wax and oil are carefully poured upon it. The slide is now 

 turned over, and the dark embryo can be seen through it 

 clearly separating itself from the white mass of wax. It is 

 easy, by turning the readily moved wax surface, to get the 

 embryo parallel with the length of the slide. This I have 

 always done with as much accuracy as possible under the 

 lens. The slide is now again to be turned over, and the 

 mixture of wax and oil is to be added until a surface is obtained 

 covering the whole of it. This surface, as soon as it is cooled, 

 is separated from the slide, and this succeeds very well if the 

 latter was thoroughly wetted with oil of cloves. The position 

 of the embrvo can now be accurately tested under the micro- 



f . 



scope ; and if it does not fully agree with the axis of the wax, 

 it can be corrected by cutting the latter parallel to it. Then I 

 carefully prick the wax surface on both sides of the embryo in 

 order to mark the position of the latter. In the next place a 



