1886. ] Microscopy. 201 
logical definition. In short, his task is no less than that of dis- 
covering, by chemical means, promorphological conditions, which 
shall reveal the destination of cells before nature has given them 
any definite histological stamp. The means that suffice to demon- 
strate fully formed tissue elements are not always identical: with 
those required in tracing their histogenetic development. As yet 
we know very little about the capacity of different preservative fluids 
in the very important work of developing nascent histological 
distinctions. It is often at the expense of much time and patience 
that reagents are found which combine the first two qualifications 
we have mentioned, and the experimenter who has been so far 
successful too frequently flatters himself that he has reached the 
highest rung in the ladder of technical bliss, if his preparations 
admit of being “sliced like cheese or cartilage.” But one re- 
quires no very large amount of knowledge of the aims, and expe- 
rience in the ways and means, of embryological research, in order 
to understand that the investigator’s art does not culminate in 
sections of cheese-like homogeneity. To be able, through serial 
Sections, to lay bare each individual cell of a complicated organ- 
ism is certainly a great triumph in microtomy; but such a feat 
may be, as it not infrequently has been, accomplished without 
leading to any important results, and simply because the methods 
of preparation have not been selected with a view to secure the 
needed differential effects. : 
. Having defined a special aim,in the use of embryological 
methods, it remains only to consider the practical side of the sub- 
ject. The differential effects of most preservative fluids, when used 
singly, are extremely weak, and often quite inappreciable. To be 
of service, they must be strengthened or reinforced by some happy 
combination of reagents, discoverable only by experiment. Dif- 
ferential results are generally sought for through metallic impreg- 
nations and through various methods of staining, as double 
Staining, multiple staining, overstaining followed by partial decol- 
Oration, etc. But I am not aware that such means alone are suf- 
ficient for the special purpose under consideration. In order to 
‘emonstrate differences, not of form, but of molecular constitu- 
tion, the foundation for the desired effects must be laid in the pro- 
cess of hardening. Staining reagents may then serve to complete 
the work. : 
As an example of what may be accomplished in this way, I will 
give briefly my own experience with osmic acid and the so-called 
Merkel s fluid, which is a mixture in equal parts of chromic acid 
(% P. c) and of platinum chloride (1% p.c.). I have tested these 
reagents with three different classes of eggs, and have obtained im- 
portant results, some of which have already been published. In 
the case of pelagic fish-eggs, with which my first experiments were 
made, the method of procedure is as follows: The eggs, with a lit- 
sea-water, are placed in a watch-glass; then, by the aid of a pi- : 
