While working at the histology of the tooth, it occurred to me that some light might be thrown on the vexed question as to the manner in which dentine grows, by observing what parts of a tooth are stained when an animal is fed on madder mixed with its food. In order to investigate this I examined the persistently growing teeth in three young rabbits, all of the same litter, which had been fed on madder (Rubia tinctorum) for two weeks. The first rabbit was killed immediately after being thus fed; the second was killed after feeding for two weeks on ordinary diet subsequent to the madder food; while the third was killed three weeks after feeding with, madder had been discontinued.