21 CUTTING . \ A I) MOUNTING TISSUES. 



in place, while it does not in any way interfere with the staining. 

 It is advisable that all paraffine sections be mounted in l>;ils;un. 

 The use of the paraffine method is recommended for the firmer 

 tissues that cannot be held in place by the collodion. 



The objections to the paraffiue method are that heat is em- 

 ployed in infiltrating and this i injurious in some degree to most 

 tissues. Further the length of time required to get the tissue in 

 condition to section is quite extended, and this is often an impor- 

 tant consideration. The operations requiring paraffine are not as 

 clean in the hands of most students as would be desired. 



The Collodion Method. 



This method is now coming into general use for nearly all 

 kinds of plant tissue. For the use of collodion for infiltrating we 

 are indebted to Duval, who first published his results in the 

 Journ. de 1'Anat., 1879, p. 185. A little later Merkel and 

 Schiefferdecker suggested the use of celloidin, which is merely a 

 patent collodion. Some discussion then arose regarding the rela- 

 tive merits of each, but it is generally agreed that one has little or 

 no advantage over the other. 



The method as applied to plant tissues is a comparatively new 

 one and many modifications of it are at present recommended by 

 various workers. 



The following directions are in the main taken from a report 

 of the author on the method, made before the Am. Soc. of Micro- 

 scopists and published in their proceedings in 1890. 



The tissue to be treated is first dehydrated and hardened in 

 alcohol. For this purpose a Schulze's apparatus is of the first im- 

 portance, in fact it has been found that some tissue can be har- 

 dened in no other way without shrinking. With its use, from 12-24 

 hours is sufficient for hardening and dehydrating any plant tissue. 

 The material is taken from the dehydrating apparatus and placed 

 in 95 per cent, alcohol for one hour, to insure complete dehydra- 

 tion. There is then poured over it a 2 per cent, solution of col- 

 lodion. In this it is allowed to remain from 12-24 hours, depend- 

 ing on the nature of the tissue, 24 hours being enough for the very 

 firmest. It is then transferred for the same length of time to a 5 

 per cent, solution; or the 2 per cent, solution may be allowed to 



