ERUPTION OF THE PERMANENT TEETH 137 



abstracts. Boas published the following results recently with 

 the remark that they ''are not very accurate:" 



I do not know how the calculations were made by Boas but 

 when compared with my results of the time when approximately 

 50 per cent of the teeth are erupted, his records show a retarded 

 eruption of all the teeth except the premolars, which are preco- 

 cious and erupt more than a year earlier than the premolars of 

 the Germans and Americans of Ann Arbor. The canines in his 

 records erupt about two years later than the 'bicuspids' (pre- 

 molars?), which is very unusual. Boas and Wissler also place 

 the end of eruption of the permanent teeth as follows: First 

 molars, 9 years; median incisors, 12 years; bicuspids, 6 to 12 

 years; canines, 6 to 15 years; lateral incisors, 15 years; and 

 second molars, 7 to 15 years. So far as I am aware, they are the 

 only records that place the eruption of the canines after the bicus- 

 pids and the lateral incisors after both. 



THE LAW OF ALTERNATION IN DEVELOPMENT 



Donaldson (17-22), Jackson (32-34), Hatai (27, 28) and others 

 have demonstrated the alternate periods of development of the 

 parts of the body without stating a law that would apply to this 

 phenomenon, and I have simply added to their work the results 

 of my observations and from the combined data I have deduced 

 a law which may be formulated tentatively somew^hat as follows : 



There are one or more periods of acceleration alternating with 

 periods of retardation in the development of each structural unit 

 or organ of the body. The periods of acceleration in the development 



