ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 155 



of muddy water to sink to tlie bottom, the larger particles collecting 

 at or near the slit mn. By this meaus the water in which the dissec- 

 tion is carried on is kept pure ; the debris is removed, and the 

 annoyance of having constantly to renew the water is avoided. The 

 supply of fresh water of course is regulated in accordance with the 

 necessity for renewal. The trough may be made of any length and 

 shape to suit the nature of the dissections to be performed in it. It 

 might be an advantage to have the sides sloping outwards instead of 

 vertical. Wrist-supports might also be fitted on to the sides. 



The trough was made for me by Mr. Frazer, optician, of Edinburgh, 

 who suggested various mechanical improvements during its con- 

 struction." 



Differentiating Embryonic Tissues.* — It may be safely assumed 

 that all hardening and staining fluids possess, in a higher or lower 

 degree, the power of developing, in the photographer's sense, histo- 

 logical distinctions between embryonic cells, long before these dis- 

 tinctions become manifest in perceptible morphological differences. 

 It is evident also, that this differentiating action varies in strength 

 according to the conditions under which the reagents are applied. 

 One of the best ways of intensifying the differential effects of hardening 

 fluids, is to use several of them in combination or in sequence. The 

 use of osmic acid, followed by Merkel's fluid, is an example of this 

 kind. The advantages of this method in the study of pelagic fish 

 eggs have already been noticed,^ and Dr. C. 0. Whitman now de- 

 scribes what the method will accomplish when applied to the eggs of 

 Clepsine. The mode of procedure is as follows : — 



The eggs are placed in 1/4 per cent, solution of osmic acid for ten 

 minutes, then rinsed in clean water and transferred to Merkel's fluid 

 (platinum chloride 1/4 per cent., and chromic acid 1/4 per cent, in 

 equal parts), in which they are allowed to remain one and a half 

 hours. They are next washed in flowing water for the same length 

 of time, then treated with 50 per cent, and 70 per cent, alcohol. 

 They need remain only a short time in the first grade of alcohol 

 (about thirty minutes), but should be left for twelve to twenty-four 

 hours in the second. For staining the author used Grenacher's 

 alcoholic borax-carmine, adding to it from one-third to one-half its 

 volume of glycerin. The glycerin intensifies the action of the dye, 

 so that a moderately deep stain is taken in the course of twenty-four 

 hours. 



It is best to stain immediately after the eggs have remained the 

 required time in alcohol, as receptivity for the staining fluid diminishes 

 considerably with the lapse of time. The osmic acid has time to 

 penetrate to all parts of the embryo, and the blackening is arrested 

 and partially removed by the action of Merkel's fluid. The diffe- 

 rential effects of the osmic acid are, however, sharpened under the 

 influence of the chrom-platinum solution. 



* Amer. Natural., xix. (1885) pp. 1134-5. 



+ Ibid., xvii. (1883) p. 1204, and Proc. Amer. Acad. Arts and Sci., xx. (1884) 

 p. 28. 



