2 INTRODUCTION 



it may almost be said that a good training in histology is one 

 of the worst backgrounds possible for cytochemical work. In 

 the type of histology and cytology which is based on showing op- 

 timal fixation, optimal staining, and optimal methods of treat- 

 ment with stains, a tremendous amount depends upon the judg- 

 ment of the investigator as to what he wishes to stain and just 

 how he wishes to see it. In some sense, this type of histology 

 and cytology is an art. On the other hand, the object of the 

 cytochemist is to use fixatives and cytochemical methods in 

 such a way that a specific and exact treatment is applied to a 

 tissue, in a manner which is predetermined by the physical 

 properties of the specimen and a precise programme of chemi- 

 cal treatment. During the carrying out of this programme, the 

 judgment of the investigator must be suspended in the interest 

 of maintaining the precision of treatment. It is only after the 

 full processes of physical and chemical treatment have been 

 carried through that the investigator can allow his opinion to 

 operate. I feel obliged to add that the standards of cleanliness 

 of reagents which are frequently adequate for classical histology 

 are, in my experience, frequently inadequate for cytochemical 

 purposes. So much is this so that, whenever I hear that an in- 

 vestigator has failed to obtain success with a cytochemical 

 method, my first piece of advice is always that he should throw 

 away every reagent that he has been using and make up a new 

 set of reagents; it is surprising how frequently this elementary 

 step is successful. 



In the second half of the nineteenth century there was a con- 

 siderable wave of interest in cytology and histology; this was 

 enabled to come to fruition by the discovery of synthetic dyes 

 which could be used for staining fixed material. It is quite clear 

 from the literature of this period that the investigators were 

 equally concerned to obtain information about both the physio- 

 logical and the chemical organisation of protoplasm. Their 

 success, however, was very largely limited to the morphological 

 and physiological side: there was an almost complete failure 

 on the chemical level. Amongst the reasons for this was the 

 fact that affinity for a given stain is only to a limited degree 

 determined by the detailed chemical constitution of the material 

 being stained. It is much more markedly determined by the 

 physical properties of the material. Difficulties were also en- 



