92 MICROSCOPICAL TECHNIQUE. 



fluid, such as the white of a hen's egg, or a | per cent, salt solu- 

 tion. One requires for such experiments a small quantity (5 — 10 

 ccm.) of freshly prepared white of egg each day. This is prepared 

 by filtering, in an uncut state, through a wad of cotton. The 

 preparation must be perfectly clear. The egg, in the morula or 

 blastula stage, is first stripped of its gelatinous envelope, and 

 placed on a circular glass plate, about 3 cm. in diameter ; then 

 covered with about 5 drops of the prepared white of egg, and torn 

 open with two dissecting needles ; or, after puncturing with one 

 needle, cut with a small curved pair of scissors. The outflowing 

 parts of the egg are then cautiously reduced in size by a few move- 

 ments of the needles. The circular plate is then placed in a 

 round glass dish (4 — ^5 cm. in diameter), with a rim i cm. high, 

 containing 10 — 15 drops of water — just enough to fill the space 

 between the edge of the object-plate and the rim of the dish, but 

 not enough to come in contact with the white. The purpose of 

 the dish and the water is to check the evaporation of the medium 

 in which the egg lies, and thus to guard as far as possible against 

 concentration of the medium and currents in the same. The dish 

 offers the advantage that one, on interrupting the observation, can 

 cover it and so protect the preparation against evaporation. Thus 

 protected, cells may be kept alive in a suitable medium for one 

 or two days. The preparation should be immediately examined, 

 while in the dish, with a low objective (e.g., Zeiss, A). It is impor- 

 tant that the table of the microscope and the object-plate bearing 

 the preparation should be perfectly level. The examination of 

 isolated cells in an uncovered medium has the advantage that one 

 can easily change the position of the cells with needles or other 

 means. But it is indispensable for checking results to examine 

 also preparations covered with a cover-slip. The cover-slip for 

 this purpose must be large enough, so that at least two of the wax 

 feet (f mm. high) supporting it may fall on dry points of the 

 object-plate, where they will firmly adhere and not allow the cover 

 to slide. A still more complete protection against currents in the 

 medium may be had by having a moist chamber ground into the 

 object-plate, and covered with a large cover-slip. The bottom of 

 the chamber must be flat and horizontal. 



After separating the cells of an egg, one searches at first with 



