THE DEVELOPMENT OF THE TEETH. 119 



the eminence of mucous membrane connective tissue which 

 constitutes the earliest dentine germ, and in section is of 

 the form shown in the figure. The surface next to the 

 dentine papilla consists of elongated columnar cells, with 

 nuclei near to their attached extremities, while the rest of 

 its substance is made up of much smaller cells, some of 

 which have inosculating processes, so that they constitute 

 a sort of finely cellular connective tissue, very different in 

 appearance from anything met with in mammalian enamel 

 organs. It is sufficiently consistent to keep up the con- 

 tinuity of all the enamel organs, even when displaced in 

 cutting sections, so that the whole might be described as 

 forming one composite enamel organ. The columnar cells 

 already alluded to invest the whole surface which is 

 directed towards the forming teeth, but they atrophy some- 

 what in the interspaces of the tooth germs. 



Before proceeding further in the description of the deve- 

 lopment of the tooth germs, it will be well to refer to a 

 somewhat earlier stage in the growth of the Dog-fish, in 

 which the relation subsisting between the teeth and the 

 dermal spines is still well seen. 



On the lower jaw of the young dog-fish there is no lip ; 

 hence, as is seen in the figure, the spines which clothe the 

 skin come close to the dentigerous surface of the jaw. 



Although there are differences in form and size, a glance 

 at the figure will demonstrate the homological identity of the 

 teeth and the dermal spines. As the dog-fish increases in 

 size, this continuity of the teeth with the dermal spines on 

 the outside of the head becomes interrupted by an extension 

 of the skin to form a lip ; this happens earlier in the upper 

 jaw than in the lower, and at first the spines are continued 

 over the edge and the inside of the newly formed lip from 

 these situations, however, they soon disappear. In structure, 

 the teeth and the dermal spines are, in many species, very 

 closely similar ; the latter are, however, much less often 



