No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 371 



haematoxylin, used by Conklin. Lithium carmine proved a good 

 nuclear stain when the eggs were first overstained, and the 

 color extracted by a long treatment with acid alcohol. Haema- 

 toxylin has the disadvantage, for the later stages of cleavage, of 

 staining very intensely the small globules of albuminous mate- 

 rial, which become very numerous and render the nuclei difficult 

 to observe. 



By far the most useful reagent that was employed was silver 

 nitrate. In fact, were it not for the beautiful and clear prepa- 

 rations obtained by using this stain, I should probably have been 

 unable to follow the cell lineage of Planorbis to the stages here 

 described. The eggs are teased from the capsules directly into 

 a .75/0 solution of silver nitrate, and exposed in a watch glass to 

 the sunlight, the brighter the better. The eggs may be exam- 

 ined from time to time with the microscope, and when the cell 

 boundaries stand out clearly, the nitrate is removed and the eggs 

 washed quickly in water. The water being mostly removed, a 

 few drops of a YJo solution of hyposulphite of soda is added and 

 allowed to act only three or four seconds. In fact, I begin to 

 remove the hyposulphite as soon as it is poured on the eggs. 

 The object of this treatment is to prevent after-blackening of 

 the eggs by dissolving out the unreduced silver. Otherwise the 

 eggs are liable to become in time so dark that they are useless 

 for study. A too prolonged treatment with the hyposulphite, 

 on the other hand, will destroy the silver stain entirely. I have 

 found it unsafe to wash the hyposulphite out with water. The 

 eggs often suddenly swell to twice their normal volume and are 

 thereby spoiled. Instead of water, a saturated solution of picric 

 acid may be used. This acts at the same time as a fixative and 

 does not injure the stain. After a few minutes' treatment with 

 this reagent, the eggs may be passed through the grades of 

 alcohol, cleared in xylol, and mounted in balsam. 



I have used to support the cover glass strips of paper of the 

 proper thickness gummed to the slide. By moving the cover, 

 the eggs may be rolled so that they can be studied from all 

 sides. When the balsam becomes hard, it can readily be soft- 

 ened by applying a drop or two of xylol to the edge of the cover 

 glass. 



