PREFACE 



TO THE 



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FOURTH EDITION. 



THE short period of three years that has elapsed since the 

 publication of the last edition of this work has not brought 

 with it any radical change in the methods of histological 

 research. Such progress as has been realised has consisted 

 rather in improvements in the detail of already-established 

 methods than in the introduction of new methods or new 

 reagents. Nevertheless, the pi-esent edition has undergone 

 a most thorough revision a revision indeed so thorough as 

 to amount to extensive re-writing in many parts. 



It has seemed to me advisable in the interest of the be- 

 ginner, and indeed in the interest of readers who are not 

 beginners at all, to enter more fully than was hitherto done 

 into the detail of the more important processes, to explain 

 more fully the principles on which they are founded, and to 

 add in many cases a critical estimate of their rationality and 

 practical value. In consequence of this re-writing, and in 

 spite of strenuous efforts to keep down the bulk of the work, 

 it has turned out to be considerably increased. I regret, 

 however, this increase the less, in so far as it is due rather 

 to the ampler treatment that has been accorded to the more 

 valuable methods than to increase in the number of processes 

 described. The number of new processes described is in fact 



