86 W. H. F. ADDISON AND J. L. APPLETON, JR. 
the apex of the tooth at an angle of from 50 to 54° with the 
dentine surface. The continuations of these rods in the outer 
layer are not so distinctly seen, but the inclination, as made 
out in thin sections, is still greater towards the apex, forming 
an angle of from 20 to 25° with the plane of the surface of the 
dentine. As noted before in the study of enamel, the rods in 
the inner layer, when observed in cross-sections of the tooth, 
decussate at an angle of from 70 to 90°, but when they reach 
the outer layer all run parallel. The fact that the rods run in 
these various directions seems incontrovertible, but in the 
light of our present knowledge of enamel formation it is difhieult 
to understand how this condition is arrived at. If each amel- 
oblast is responsible for an enamel-rod, then it follows that 
because the alternate layers of rods are oblique to one another, 
the ameloblasts responsible for these series of rods must have 
changed their relative positions during the process of formation 
of these rods. No such phenomenon has been observed, or 
even suggested. The other possibility is that the matrix of 
the rods is formed in a regular manner, but that afterwards, 
before calcification is complete, the rods become re-arranged 
owing to pressure strains. 
The plane of direction of the rods is suggestive of the im- 
portance of the enamel-organ in the persistent growth. For 
always the general plane of the rods, as they leave the enamel- 
dentine junction, is towards the outer end of the tooth, as if 
the ameloblasts, while engaged in enamel-formation were al- 
ways held back by the enamel, in which their processes were 
imbedded. 
The basal formative end of the enamel-organ (region 1) in 
the adult animal corresponds very closely in structure with 
what has already been described for earlier ages, e.g., 4 days. 
This is the region where the enamel-organ is constantly being 
renewed, and it retains the same embryonal character at all 
stages of development. 
At the anterior end where the enamel-organ is continuous 
with the surface epithelium (region 2), a gradual transition oc- 
