G. GENERAL NATURAL HISTORY AND ZOOLOGY. 291 



and relationships ; and it may interest them to learn of the 

 experiences of Professor Van Beneden in solving this problem. 

 He not long since exhibited before the Academy of Sciences 

 of Belgium preparations of specimens of various kinds of 

 Medusae,, Cte?iopho?'ce, etc., which had been prepared for sev- 

 eral weeks, and were remarkable for the excellence of their 

 condition. Two different substances were made use of for 

 the purpose in question ; one a weak solution of osmic acid, 

 and the other a solution of picric acid. Osmic acid has of 

 late come into extensive use in histological investigations. 

 It has the property of hardening the tissues of the more deli- 

 cate organs, so as to allow very thin sections to be made for 

 microscopical purposes. It has also the valuable peculiarity 

 of coloring, first brown and then black, the fatty matters in 

 general, particularly myeline. It tinges the epithelial cel- 

 lules brown, as well as the muscular elements. It brings out 

 distinctly the fibrillium of the cylinder of the axes of the 

 nervous fibres, showing the isolated nervous fibrils, and gen- 

 erally defining the limits of the cellules, and showing their 

 different characteristics. To use this osmic acid in preparing 

 Medusm and Ctenophorm, so as to keep them from the de- 

 structive agency of alcohol, they are to be treated in a feeble 

 solution of the osmic acid (one sixth to one tenth per cent, to 

 one hundred of water) for a period varying, according to the 

 nature of the objects, from fifteen to twenty-five minutes. 

 After this lapse of time the animals assume a very slight 

 brownish tinge, the cellules of the endoderm and the organs 

 formed at the expense of the endodermic sheet alone becom- 

 ing colored ; the other tissues preserve their primitive trans- 

 parency. Thanks to this property, the endodermic cellules 

 and the extra vascular canals become admirably defined, 

 while the cirrhi are more distinct than in the living medusa. 

 At the same time all the tissues become hardened, and the 

 objects can then be removed from the acid solution ; and 

 after being carefully washed, they are to be placed in strong 

 alcohol, without any danger of their ultimately losing either 

 their elegance or the transparency of their tissues. Indeed, 

 the organization and structure of these delicate objects can 

 be studied weeks after, and perhaps even months, as well as 

 if they were living under the eye. 



Another method which Professor Van Beneden has em- 



