DEVELOPMENT OF THE ENAMEL 153 



separate structures or are formed by the union of the 

 limiting membrane or wall of the cell, has not been deter- 

 mined. That in the case of both the outer and inner mem- 

 branes, processes pass through them from the cells, both of 

 the stratum intermedium and the ameloblasts, there can 

 be little doubt, but the protoplasm of the cell wall is not 

 a rigid substance like a sieve, and is quite capable of acting 

 as a dialysing membrane, although these processes do pass 

 through it. 



Both Tomes and Carter consider it an inexplicable 

 anomaly that the dialysing membrane should be penetrated 

 by the fibrils of the Tomes' processes ; but while Tomes 

 denies the existence of such separating membrane, Carter 

 apparently allows the existence of some such structure, but 

 not the processes of the cell, as he considers with Tomes 

 that Ave could not have a dialysing membrane which is 

 perforated by processes. In this connexion it may be 

 mentioned that Ramon y Cajal l describes the intercellular 

 bridges of epithelial cells as being covered with a prolonga- 

 tion of the cell membrane. 



In order to avoid the distortion of the delicate tissues of 

 the enamel organ so frequently produced by the employ- 

 ment of such reagents as alcohol and balsam, and the 

 employment of heat in the process of embedding, the author 

 in his investigations of enamel development made use of 

 preparations hardened in formol and cut in the freezing 

 microtome. These were stained with watery solutions of 

 the dyes and mounted in glycerine or Farrant solution. 

 By this method preparations were obtained which showed 

 all the parts of the enamel organ in their proper relations 

 to one another and free from the shrinking and distortion 

 so often seen in preparations of tooth-germs. 



The first notable point brought out in marsupial germs 

 prepared in this way is the great distinctness of the Tomes' 

 processes of the ameloblasts, as was pointed out by C. S. 

 Tomes (figs. 84 and 85). They are not drawn out into thin 

 lines or apparently pulled apart from the forming enamel, 

 and are seen in many places to be nearly as wide as the 

 cells from which they arise. C. S. Tomes says that these 

 1 Int. M onatschr. f. Anat. u. HistoL, iii. 1886. 



