Vlll FOREWORD 



by the vascular bundles, enabled the worker to obtain sections with- 

 out previous preparation. The introduction of the freezing-drying 

 technique by Gersh removed to some extent this advantage and 

 enabled the worker to obtain sections of animal material without 

 previous chemical treatment. This valuable method has been wisely 

 chosen as the first subject of the first section. 



This section is devoted to the methods for recognition of chemical 

 substances in microscopic preparations. The author has contented 

 himself with presenting accurately and without prejudice the many 

 methods so far suggested for the detection of various chemical sub- 

 stances in tissues. In this field there are three purposes to be served 

 — namely, recognition, localization, and quantitation. The first of 

 these may be accomplished equally well by macrochemical methods 

 and the third has only recently achieved importance through the 

 introduction of the newer physical methods of measurement. Accord- 

 ingly, localization becomes the chief function of microchemistry of 

 this order. On the user rests the responsibility for perceiving and 

 avoiding the many pitfalls which are inevitable. Gross blunders have 

 been made in the past and can only be avoided in the future by 

 meticulous care and high critical ability on the part of the worker. 

 The belief that in the complex colloidal matrix of protoplasm reac- 

 tions occur as they do in more simplified systems in vitro is respon- 

 sible for many mistakes. No assumption as to solubility or insolu- 

 bility is admissible. Otherwise soluble substances may be firmly 

 adsorbed to submicroscopic surfaces or incorporated into molecular 

 aggregates. In those methods which require fixing, imbedding, and 

 cutting in order to prepare the material for microchemical tests, the 

 disparity between the mass of material and the volume and variety 

 of solvents employed makes the data on insolubility equally untrust- 

 worthy. The worker must examine critically each step of the process 

 and seek to api)raise its effect on the final result. Reaction time, drift 

 of highly dispersed reaction products to other locations due to surface 

 charges, and translocation of reaction owing to differences of ion 

 mobility, all must be carefully considered. 



Microchemical reactions yielding colorless products and requiring 

 the use of a second reagent to visualize them should be accepted only 

 with reservation and should be used only as a first approach, subject 

 to the results of later confirmatory tests. To this category belong, 

 for example, the molybdate reactions for the detection of phosphate, 



