FIXING AND HARDENING AGENTS. 47 



These are mixed, and after a short time give a fine violet- 

 coloured solution. 



The objects are immersed for four to five hours, and then 

 passed through 70 per cent, alcohol (twenty-four hours), 

 strong alcohol (some clays), absolute alcohol (four to five 

 days). They are then fit for cutting. The advantage of 

 the process is said to be, amongst others, that segmentation 

 spheres and nuclei are perfectly fixed, the ova do not become 

 porous, and cut like cartilage. 



For a special formula for embryological purposes, see 

 the paper quoted. 



This liquid has been for a long time in great vogue not 

 only for embryological purposes, but for general work and 

 cytological work. But opinions are divided as to its merits. 

 I myself have extensively used it for preparing specimens 

 for dissection and for museum specimens, and have found it 

 admirable for these purposes. But specimens made to test 

 its value from a cytological point of view have given me only 

 second-rate results. 



MAYER contributes the following note on this subject to 

 the Grundzuge (p. 34) : " Pereiiyi's mixture does not appear 

 to have been hitherto considered from a chemical point of 

 view. It is, however, easy to see that as soon as the mixture 

 has become violet, the chromic acid no longer exists in it as 

 such, but has been changed into chromic oxide. At the 

 expense of this oxide the alcohol becomes oxidised, and in 

 consequence of the presence of the nitric acid becomes 

 partially converted into nitric ether. Consequently the 

 liquid is reduced essentially to a mixture of alcohol of at 

 most 30 per cent, strength with about 5 per cent, of nitric- 

 acid. An analogous mixture made by omitting the chromic 

 acid preserves, according to my experiments, in just the 

 same way as Perenyi's, that is, just as so weak jin acid 

 alcohol can be expected to preserve, and that is rather ill 

 than well. Objects, it is true, do not shrink in it ; indeed, 

 they rather swell, sometimes to a marked degree. And, in 

 fact, observers are not wanting who entirely reject it for the 

 fixation of ova. See the strange results of Cholodkovsky in 

 the embryology of Blatta, on which I have commented in 

 Zool. Jahresbaricht, 1891 (Arthropoda, p. 61), and which 

 Wheeler and Heymons have later (ibid., 1893, p. 71, and 



