ZOOLOGY. 
THE PRESERVATION OF THE LOWER ANIMALS.— I have the honor 
of making known to the class two methods that I have employed 
at Helgoland, during the last season, for the preparation and pres- 
ervation of Medusæ, Ctenophoræ, Noctilucæ and most of those 
lower forms, transparent as crystal, which live at the surface of 
the sea, and which the use of the towing net furnishes in abun- 
dance. I submit to the class different Medusæ (Oceania, Gery- 
onopsis), Ctenophore and some Noctiluce prepared for several 
months, and remarkable for their perfect preservation. 
One of these methods consists in the use of a weak solution of 
osmic acid, the other in the use of picric acid. 
Osmic acid has been constantly employed in histology, espe- 
cially for the study of the nerve-terminations, and Max Schultze 
has made known, by his beautiful researches on the structure of 
the retina, all the advantages which the use of this reagent 
presents. Not only does the osmic acid harden the-most delicate 
tissues and organs, allowing us to make fine sections, but it 
possesses the valuable property of coloring brown, afterwards 
black, the fatty parts in general, and more particularly myeline. 
It tints in brown epithelial cells and muscular tissues ; it renders 
very apparent the fibrillar structure of the cylinder of the axis of 
nervous fibres, and brings out clearly the isolated nerve fibrille. 
Very recently, F. S. Schultz has employed with great success 
osmic acid for his beautiful histological researches on Cordylo- 
phora lacustris. This reagent indicates admirably the limits of 
the cells and brings out well their different characters. 
I have used osmic acid to prepare Medusx and Ctenophore, in 
order to save them from the destructive action of alcohol, in the 
following manner. The object is placed in a very feeble solution of 
osmic acid, for a time varying, according to the nature of these mi- 
nute organisms, from fifteen to twenty-five minutes. After this lapse 
of time the animals are colored pale brown; the cells of the endo- 
-derm and the organs attached to the endodermic layer are alone col- 
~ ored, and the other tissues preserve their original transparence. 
_ Thanks to this coloration of the endodermic cellules the gastro-vas- 
cular canals are admirably indicated, and the cirrhi become more 
_ distinct than in the small, living Medusæ. At the same time all the 
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