ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 745 



warmed over a small flame until the desiccation is nearly complete. 

 The sections are withdrawn directly from the action of the flame, and 

 a few drops of water added, which dissolve the calcium chloride. The 

 sections immediately float in the water ; they need only be collected 

 and placed in the glycerin, in which they attain sufficient transparency 

 after a few hours. This treatment results, not in dissolving all that 

 the cells contain, but in darkening their contents by slightly 

 thickening the originally very thin walls ; these walls become at the 

 same time clear and brilliant. The opacity of the cell-contents 

 obstructs the study of several layers of cells at the same time. 



II. Fixation of Foems. 



The ternary parts of the plant being generally tolerably rigid, it 

 is only necessary to fix the proteid matters (protoplasm, nuclei, 

 vibratile cilia, &c.). The following agents are employed for this 

 purpose. 



Absolute alcohol. — "When absolute, alcohol fixes the protoplasm 

 without contracting it. It can be made to act directly on the prepa- 

 rations to be examined, or upon the organs before making sections. 

 Strasburger has studied in the latter mode the formation of the cells 

 in Iris pumila. By immersing Spirogyra orthospira in absolute 

 alcohol at different hours of the night he succeeded in fixing the 

 different phases of the division of the nucleus in this alga, which it 

 then became very easy to study by daylight (without its changing) 

 the day after and the following days. The same observer succeeded 

 in retarding division until the morning by placing the Spirogyra in a 

 room without heat in November. He was thus able to follow under 

 the Microscope all the phenomena of the division, and to fix them at 

 the most suitable moment by immersing the plant in absolute 

 alcohol. 



CJiromic acid. — L. Guignard has successfully employed chromic 

 acid to fix the nuclei in the embryo-sac in the Mimosae.* The 

 good results he obtained with it mark this reagent as one of the 

 most valuable in vegetable microchemistry. 



Osmic acid. — Osmic acid, whilst fixing the form, has the advantage 

 of giving transparency to the protoplasm and the cell-walls, but 

 has also the inconvenience of destroying the protoplasm after some 

 hours. Strasburger has nevertheless used it in his observations on 

 the division of nuclei. He placed the plants in water containing 

 l-500th of sugar, and added one or two drops of a 1 per cent, 

 solution of osmic acid. 



Vignal j" and Certes + have called the attention of naturalists to 

 the good results obtained with osmic acid for fixing instantaneously 

 the forms of the lower organisms {Noctilucoe, infusoria, algse, zoo- 

 spores, microbes of virulent diseases, &c.). Generally it is sufficient 



* Bull. Soc. Bot., 25tli June, 1880. 



t " Kecherches histologiques et physiologiques sur les Noctiluques," Arch, de 

 Physiol., 1878. 



X " Sur une methode de conservation des infusoires," Comptes Kendus, 3rd 

 March, 1879. 



