356 THE FORMATION AND STRUCTURE 



and still hold, widely dififerent views. This difference, I believe, 

 has largely grown out of relying too closely on one or on a few 

 particular methods of working. I have seen many illustrations of 

 the possibility of drawing contradictory conclusions from appear- 

 ances presented by different methods of treating tissues, each of 

 these conclusions very probably representing a half truth. Another 

 reason may also be found in the very great difficulties attending 

 the preparation of thin sections of developing teeth with all of 

 the tissues in their normal relations. In deciding, therefore, to 

 reinvestigate the whole field of the development of the teeth, I 

 determined at the outset to adopt a large number of methods of 

 working, in the hope that, by comparing the results of widely 

 differing treatment, T might unravel some of the tangles in which 

 the subject has always been involved. I selected for the com- 

 mencement of my work embryos of the cat, dog, sheep, cow, hog, 

 and monkey, as well as human embryos of all stages of develop- 

 ment. The rodents were also represented by mice, rats, and 

 guinea-pigs. Some of the material was cut while fresh, and some 

 immediately after fixing by freezing processes. Other material was 

 fixed and hardened by a great variety of processes, and embedded 

 in balsam by the Weil process, or decalcified by different acids and 

 embedded in paraffin and celloidin. 



More than thirty different stains and combinations of stains 

 have been tried, and sections have been mounted in balsam, 

 Farrant's medium, glycerol, sulpho-carbolate of zinc in glycerol 

 (my favourite medium for purposes of study), and other preserva- 

 tives. As there is but little difference of opinion concerning those 

 earlier stages of tooth development which precede the commence- 

 ment of the formation of dentine and enamel, we need give no 

 further attention to this period than is necessary for an introduction 

 to the subject proper. All students of dental histology are now 

 aware that the teeth are developed from two distinct tissues. The 

 so-called enamel organ is a product of the oral epithelium, while 

 the dentinal germ or papilla is formed in the submucous or dermal 

 tissues, the former being the first to appear. 



The first sections cut for these special studies were from the 

 jaw of an embryo Iamb at a period just after the commencement 

 of enamel formation. The material was fixed in a saturated 



