202 Generat Notes. [ February, 
ette, a quantity of osmic acid ( 14 p. c.) equal (as nearly as one can 
judge) to that of the sea-water isadded. At the end of from five to 
ten minutes, the eggs are washed quickly in clean water, and trans- 
ferred to a chrome-platinum solution, differing from Merkel’s mix- 
ture only in having a higher per cent of chromic’ acid, where they 
may remain from one to three days. After this treatment, the 
blastoderm may be easily freed from the yolk, and, after a thor- 
ough washing in clear water for a number of hours, the prepara- 
tion may be passed through the usual grades of alcohol, stained 
and sectioned, or mounted in toto.. The osmic acid fixes the 
natural form and structural features of the egg perfectly, and the 
mixture of chromic acid and platinum chloride completes the 
work of hardening, and at the same time removes much of the 
brown or black color imparted by the first reagent. I have tried 
various other reagents after the osmic acid, but with far less 
satisfactory results. Picro-sulphuric acid, instead of arresting the 
blackening process of the osmic acid, increases it. Simple 
chromic acid arrests the blackening, but does not remove it (as 
does Merkel’s fluid), and causes considerable contraction. Mil- 
ler’s fluid, recommended by Henneguy, is equally unsatisfactory. 
By this method a very marked differentiation is generally ob- 
tained as early as the sixteen-cell stage, the four central cells 
showing a very light brown shade, while the twelve peripheral 
cells have a much deeper shade. In later stages of cleavage, the 
distinction between central and marginal cells becomes still 
stronger, so that it becomes possible to trace the entire history of 
the origin of the so-called parablast, over which there have been 
so many controversies. The very difficult question as to the 
precise origin of the permanent entoderm is not settled by this 
method. 
The same reagents may be successfully applied to the eggs of 
Clepsine ; but here the mode of procedure is somewhat different, 
as regards Merkel’s fluid. This mixture, employed at its normal 
strength, is allowed to work from one to two hours only. The 
differential effects are here very marked, extending not only to the 
different germ-layers, but even to cell-groups destined to form the 
central nervous system, the nephridial organs, larval glands, etc. 
None of the methods hitherto employed with these eggs has 
given results at all comparable with those I have mentioned. 
In the case of the frog’s eggs, I allow the osmic acid from 
twenty to twenty-five minutes, then transfer directly to the 
chrome-platinum solution employed with fish-eggs (twenty-four 
hours). The eggs are next placed in water and freed from their 
gelatinous envelopes by the aid of sharp needles and a dissecting 
microscope. After washing in flowing water for at least two 
hours, the eggs may be treated with alcohol and stained accord- 
.,} A one per cent solution is used in place of the normal x p. c. solution. 
