DEVELOPMENT OF THE TEETH IN MAMMALIA 23 



meeting over its upper surface. It thus becomes closed in 

 by the mesodermic tissue, which forms the sac of the develop- 

 ing tooth. 



The tooth-sac of the developing permanent tooth is Tooth- 

 closed in by a bony shell, except at its uppermost point, sac * 

 where it is pierced by a foramen which opens upon the 

 gum behind the corresponding milk tooth. From the sac 

 a fibrous band passes through this foramen and becomes 

 blended with the gum behind the milk tooth. This band 

 or cord is called the ' gubernaculum ' or rudder, as it was Gubema- 

 supposed to guide or direct the course of the erupting tooth, culum. 

 There is, however, no distinct canal, but bands of connective- 

 tissue fibres enclosing strands of epithelium. 



Its position is marked by the foramina, which are seen in 

 the bone immediately behind each of the temporary teeth. 

 Malassez (16), in a paper ' On the Structure of the Guber- 

 naculum dentis ', says : ' The teeth of replacement are con- 

 tained in a bony cavity which is prolonged in the form of 

 a canal to the alveolar border, where it opens at the inner 

 side of the milk tooth. The dental follicle prolongs into the 

 canal a kind of cord which continues until lost in the fibrous 

 tissue of the gingival margin.' It was considered by 

 Delabarre (5) and Serres (20) to be hollow and to guide the 

 tooth in eruption. Sappey (21) said it contained 'the last 

 remnants of the epithelial proliferations '. Malassez con- 

 siders there is no part of the gubernaculum in which a canal 

 exists, but it is made up of connective-tissue fibres, mostly 

 arranged longitudinally, but the point to which he would 

 draw particular attention is that the connective tissue 

 encloses numerous epithelial strands. In the deep part the 

 epithelial elements are more abundant and form a ' rich 

 network ' and ' lateral buds in the form of clubs ' are to be 

 seen. ' In the most superficial part of the gubernaculum 

 they are, on the contrary, less numerous, less rectilinear, 

 and more rarely anastomosed.' He also says that these 

 tracts of epithelium can be seen to proceed from the ' corre- 

 sponding enamel cord of the enamel organ ', which he 

 considers not only persists, but ' even proliferates with great 

 activity in the neighbourhood of the tooth of replacement '. 

 and his conclusion is that ' this fact leads us to surmise 



