1882.] the Enamel of the Teeth in Vertebrates. 



161 



there is generally a faint granular striation running from the surface 

 of the pulp to the basement membrane. 



Fig. 9 is a portion of a vertical longitudinal section of the young 

 tooth of a thornback hardened in alcohol. The basement membrane 

 could be traced from the derma quite up to the apex on one side ; it 

 was quite invisible upon the other side, which was cut slightly 

 obliquely to the surface of the tooth. The series of sections, how- 

 ever, furnishing the requisite conditions showed it to be there also. 

 But in this section (fig. 9) not the slightest trace of it can be 

 detected, and, but for subsequent sections, one could declare it did not 

 exist. 



If, at any time before calcific deposit, a young tooth* be carefully 

 cut out, with a portion of the surrounding derma, and the cells of 

 the enamel organ be washed away, examination with the microscope 

 will show its outer surface to be quite smooth and dense, f and a clear 

 strong homogeneous membrane, enveloping granular and cellular 

 contents, can be pretty clearly made out. Pressure of the coverslip, 

 causing the membrane to split, will bring out still more clearly the 

 different structures of the perfectly clear granular transparent dense 

 membrane, and the enclosed cellular pulp or granular dentinal basis 

 upon its surface. If the tooth be now hardened in alcohol and cut 

 into thin sections, those which are perfectly vertical will show the 

 basement membrane of the small portion of adherent dermis to run 

 up over the tooth as shown in fig. 8. It is exceedingly delicate and 

 perfectly transparent, and when the surfaces of the tissues are cut at 

 all obliquely, it is as invisible as a thin film of glass. Hence it can 

 seldom be followed over the entire surface of even the most perfect 

 section, and it cannot, like the cuticula in a somewhat older tooth, 

 be separated by the use of acid. When a small bit is fortunately torn 

 away with needles, it shows no " cell markings " upon it, but is per- 

 fectly smooth. 



If a young tooth papilla be treated in this way, the basement mem- 

 brane will be found quite upon the cellular pulp, a few cells of the 

 enamel membrane frequently adhering to its surface. If the section 

 be teased slightly and the membrane broken, the short outlines of its 

 ends, sometimes standing out alone (Plate 4, fig. 10, b. m.), and its 

 clear dense structure appears in marked contrast with the granular 

 underlying pulp. 



When the teeth, in situ, have been treated with chromic acid for a 

 week or more, as is generally done, and then imbedded, and cut 

 very much at random as is necessarily the case, it is a great chance 

 if any trace of basement membrane can be seen. The enamel 



* A small tooth of one of the Plagiostomes answers best — say of the thornback. 

 f At this stage the cuticula, if its formation be begun, is generally brushed off 

 along with the enamel cells. 



VOL. XXXIV. M 



