Vl PREFACE TO THE THIRD EDITION 



is given followed by the address (so that questions can be asked if desired) and the 

 date (so that it can be seen at a glance when it was last checked for improve- 

 ments). To all of these friends I am very grateful. Proofs of their contributions 

 have not been submitted to them for corrections, because there are so many such 

 accounts — ^most of them very brief — and the material presented by them was 

 clear and concise. For typographical errors the responsibility is to be shared 

 by me and the publisher. Most of the text I have written or revised and in 

 doing so I have relied heavily on the reference books listed on page xxxi. 



It is hardly necessary to explain that the policy is to provide brief accounts 

 of techniques, and leads to others, and to make this information quickly available 

 by alphabetical arrangement. It is better to give some data not required by 

 experts than it is to write mainly for well-informed but limited groups. Ob- 

 viously the said experts must approach fields other than their own as beginners 

 handicapped by unfamiliarity with speciahzed technique and equipment. Be- 

 cause the several biological and medical sciences have so much to offer each other 

 in the way of laboratory technique this exposure of opportunities may facilitate 

 inter-specialty cooperation. My thanks are due to Mrs. Theresa Bresnahan for 

 help in preparing the manuscript. 



E. V. COWDRY 



St. Louis 



