380 H. E. JORDAN 



only as far as the middle of the stellate reticulum. Williams 

 concludes that in the tooth of the sheep the blood vessels pene- 

 trate as far as the stratum intermedium — a membrane surmount- 

 ing the layer of ameloblasts. Hopewell-Smith makes a similar 

 claim for the tooth of the kitten — a claim controverted by Jordan. 



The question of the significance of the stratum intermedium 

 also calls for incidental consideration in this connection. Williams 

 is of the opinion that the cells of this layer select from the blood 

 the materials out of which the enamel is elaborated. The 

 facts, however, that a stratum intermedium is not present in 

 typical form over the entire crown in certain mammalian teeth, 

 that where it occurs typically it may extend below the level of 

 enamel formation, that enamel formation begins before the com- 

 plete development of this layer (Williams) , and that it apparently 

 does not exist in the enamel organs of reptilian teeth (Hopewell- 

 Smith) cast considerable doubt upon this hypothesis. 



The material of this investigation throws light also on the ques- 

 tion concerning the function of that part of the enamel germ 

 which extends below the level of the future neck of the tooth, 

 that is, below the level of amelogenesis. Skillen accepts, and 

 seeks further to support, the hypothesis of Rose that the presence 

 of the ameloblasts is essential as a stimulative factor for the pro- 

 duction of dentin. These investigators conceive of the non- 

 amelogenic portion of the enamel organ as a 'stimulative layer' 

 for the formation of the dentin of the subcervical portion of the 

 tooth. That this hypothesis is inadequate, if not actually 

 erroneous, is proved by the fact that in the case of the incisor 

 teeth of the white rat the enamel organ never completely envelops 

 the growing tooth, dentin forming nevertheless in the absence of 

 any portion of the enamel organ over the inner surface of these 

 teeth. 



MATERIAL AND METHODS 



The material chiefly employed in this investigation consists 

 of sections of the head, in the region of the incisor teeth, of the 

 white rat of from one to seven days after birth, and of sections 

 of the lower jaw of the kitten, new-born and one, two, and three 



