494 \Yisco?hsi?i Academy of Sciences^ Arts^ and Letters. 



lots new nets apparently under twenty-four hours old were ob- 

 sen^ed. 



The difficulties in getting well fixed material of such alga 

 cells as" those of Hydrodictyon possibly help to account for the 

 fact that so few investigations have been carried on in this 

 group of plants by means of modem cytological methods. 

 The relatively thick and impervious cell walls and thin layers 

 of cytoplasm combined with a large central vacuole present a 

 set of conditions under which fixation without shrinkage and 

 consequent distortion of the plasma contents is hard to accom- 

 plish. A solution must be found that will readily penetrate 

 the cell wall and plasma membrane and thus fix the cells mth- 

 out distortion. The two solutions that I have found to best 

 answer these conditions have been ^MerkeFs platinum chloride 

 chromic acid mixture and a mixture of iridium chloride and 

 acetic acid. Two formulas were used for this latter combina- 

 tion ; one that of Eisen consisting of 100 parts, five^tenths per 

 cent iridium chloride in distilled water and 1 part glacial acetic 

 acid ; and a stronger sohition consisting of 100 parts of 1^ 

 iridium chloride in distilled water and 3 parts glacial acetic acid. 

 Very little difference could be detected in the effects of the two 

 iridinm-chloride-acetic acid mixtures, although the stronger 

 Avas generally more reliable for fixing the finer details of struc- 

 ture, especially in the nuclei. 



As between the ^lerkel's solution and the iridium chloride 

 acetic acid mixture, the latter is to be preferred for cases where 

 very delicate structures are to be dealt with, such, for example, 

 as the comparatively young cells of Hydrodictyon where the 

 layer of protoplasm is very thin and its finer structures very 

 easily destroyed. Still both solutions are veTv satisfactory and 

 I have tried them on various other Algae, including Spirogyra 

 and Yauclieria, with gratifying results. They are to be fur- 

 ther recommended on account of their adaptability to various 

 stains. I have found that the triple stain of Flemming, the 

 Fuchsin-Iodine green of Zimmerman and Ironliaematoxylin 

 all give good results. 



Flemming's cliromo-osm.o-acetic acid mixture was tried for 

 fixing but abandoned owing to the blackening of the tissue 



