374 SOME ZOOLOGICAL METHODS. 



Medusae that have not very contractile tentacles can be 

 satisfactorily killed by solution of sublimate or a chromic or 

 osmic liquid in the usual way. 



According to De Castellarnau (I. c.) Cassiopeia borbonica 

 requires a special treatment. The animals should be treated 

 with osmic acid until they begin to change colour, and then 

 be put for two or three days into 5 per cent, solution ot 

 bichromate of potash, and then into alcohol. 



745. Medusae : Sections. I am not acquainted with any per- 

 fectly satisfactory method of sectioning these extremely 

 watery organisms. Paraffin and collodion will afford good 

 sections of some organs, but are certainly not satisfactory as 

 all-round methods for this group. Some modification of the 

 method employed by the HEETWIGS (Nervensystem der Medusen, 

 1878, p. 5) might be successful. They imbedded in liver 

 with the aid of glycerin gum, and hardened the objects and 

 the mass in alcohol. I should think better results would be 

 obtained by one of the freezing methods given in 308 to 312. 



746. Medusae : Maceration. The methods of the HERTWIGS 

 (Das Nervensystem u. die Sinnesorgane der Medusen, Leipzig, 

 1878, p. 5) have deservedly become classical for the study of 

 the tissues of this group. The objects are treated for two or 

 three minutes, according to their size, with a mixture of equal 

 volumes of 0'2 per cent, acetic acid and O5 per cent, osmic 

 acid, and then washed in repeated changes of 0*1 per cent, 

 acetic acid until all traces of free osmic acid are removed. 



hey then remain for a day in Ol per cent, acetic acid, are 

 then washed with pure water, stained with Beale's carmine, 

 and preserved in glycerin. 



Amongst other advantages of this mixture it is noted that 

 the reduction of osmic acid by albuminates is greatly hastened 

 by the presence of acetic acid, which in the case of animals 

 so transparent and poor in cells as medusae is an advantage 

 for the study of the nervous system. For ganglion-cells and 

 nerve-fibrils reduce osmium quicker than common epithelimn- 

 cells. They become greenish brown, and are easily distin- 

 guished from surrounding tissues. 



The isolation of the elements of the macerated tissues is 

 best done by gently tapping the cover-glass (which may be 

 supported on wax feet). This gives far bettor results than 



