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WILSON. [Vol. VI. 



with that employed for the surface-views and optical sections, 

 and to it I owe many important results. This is simply strong 

 acetic acid mixed in various proportions with glycerine and 

 water. I have usually employed a mixture of glycerine, glacial 

 acetic acid, and water in equal parts (a modification of " Haller's 

 fluid," suggested to me by Dr. Watase). The eggs are placed 

 directly in this fluid and kept there until needed, i.e. for an 

 indefinite period. They are perfectly fixed, without change of 

 form, and undergo no deterioration for several weeks except a 

 gradually increasing vacuolation of the protoplasm. For exam- 

 ination they are stained as follows. A number of the eggs, 

 still lying in the fluid, are transferred with a pipette to a watch- 

 glass and a few drops of Schneider's acetic carmine (saturated 

 solution of carmine in forty-five per cent acetic acid) are added. 

 The proper degree of staining must be determined by exam- 

 ination ; the time required depends upon the amount of carmine 

 added. The color should be light red, and I have usually found 

 three to five minutes sufficient with a rather weak carmine. 

 The embryos are then washed by repeatedly changing tJie glycer- 

 ine-acetic fluid until all superfluous color is removed ; they are 

 then mounted, still in the fluid, under a long cover-glass as usual. 

 They may be examined immediately, but the embryos become 

 far more transparent if the preparation be set aside for several 

 hours until the water evaporates, e.g. over night. (After a day 

 or two the color begins to alter and to darken, and the prepara- 

 tion soon becomes useless.) If, now, a favorable specimen be 

 selected, slightly compressed and carefully rolled about from 

 side to side by displacing the cover-glass with a needle, the cells 

 may gradually be caused to separate from one another. If the 

 process be stopped at precisely the right point, when the cells 

 have barely begun to separate, preparations of the utmost beauty 

 and clearness may be obtained. The protoplasm is colored pale 

 red, the resting nuclei and the karyokinetic figures show with 

 perfect distinctness, and especially the cell-outlines are shown 

 with diagrammatic clearness. In good preparations, even of 

 comparatively late stages, every cell in the embryo may be seen 

 and the relations of the dividing cells studied with the utmost 

 accuracy. Most of the figures have been drawn from specimens 

 prepared by this method, and to it is owing the relative com- 

 pleteness with which I have been able to study the cleavage- 



