12 MICROSCOPICAL RESEARCH 



The reason for the use of "fixatives" has already been stated. 

 General rules only can be laid down for the guidance of students, the 

 "absolute" is gained only by actual experience. Collecting specimens 

 at the proper time and under suitable conditions, with a due regard to 

 their fixation and after treatment, is of the greatest importance in 

 histological work, as on this more than anything else depends the 

 success of the after operations, — no after manipulative skill can rectify 

 bad fixation and preservation. It is only after we have gained a 

 knowledge of the nature of things that we can decide readily as to the 

 use or non-use of fixatives, whether in any case their use is imperative, 

 or altogether unnecessary. In all cases this will be indicated when 

 dealing with the "type preparations" used to illustrate this work. The 

 action of a true fixative is clearly illustrated by the photomicrograph 

 Fig. 3 A, and an unsuitable one is shown at B. Both sections are 

 prepared from the same specimen, one being fixed with the chromo- 

 acetic solution, the other placed direct into 92 per cent, alcohol. 

 The former is perfectly fixed, the cell-contents are preserved, and no 

 contraction or shrinkage of the cell-walls has taken place. In the 

 other there has been a complete breakdown and disorganisation of the 

 entire system of cells and their contents, brought about by the rapid 

 "dehydrating" action of the alcohol on the tissues. The specimen is 

 an aquatic one, the cells of which are filled with water and the tissues 

 are too delicate to resist the action of the alcohol, hence their collapse 

 and destruction. 



On the other hand, where the vascular system of the specimen is 

 more highly developed, and when grown in dry soil, it may be plunged 

 direct into 92 per cent, alcohol without any apparent injury to the 

 tissues, as is clearly illustrated by the roots of Ranunculus and Zea 

 mais (Plate i, Figs, i and 4). 



