632 p]KNEST WARREN. 



Iw inakiug a series of preparations illustrating a course of 

 lectures on odontology I was led to the Cyclostotnes, and I 

 have cut sections through the heads of young specimens of 

 Myxine glutiuosa (L.) (about five inches long), and of 

 Petromyzon marinus (L.). These sections have not 

 confirmed the views of Dr. Beard, and there can be no doubt 

 that the cone of " odontoblasts " is purely epidermal in 

 origin, and is, in fact, a successional tooth developing be- 

 neath the functional tooth. 



The heads were stained with borax carmine, and treated 

 with acid alcohol. They were then slowl}- impregnated with 

 chloroform and paraffin, and ultimately were passed into 

 pure paraffin (58°). In the case of Mj^xlne it was necessary 

 to paint the surface of the block with a solution of celloidin 

 and gum mastic before cutting every section. Good sections 

 were thus obtained, and they were counter-stained on the 

 slide with picro-nigrosin. With this stain anything of the 

 nature of horn becomes bright yellow, while connective- 

 tissue fibres become blue. 



Petromyzon marinus (L.). — Fig. I (A, B, C) illustrates 

 the development of a horny tooth situated near the edge of the 

 mouth. In fig. A the tooth follicle, if it may so be termed, 

 has already been formed by the downgrowth of the epidermis 

 into the corium. A small mesodermal papilla, like that of a 

 hair, is present. The first sign of the developing tooth 

 occurs a little above the mesodermal papilla. The cells at 

 this place proliferate, gradually expand, and become granular. 

 The cells of the epidermis above the developing tooth become 

 flattened by the pressure exerted. 



In fig. B the young tooth is more definitely marked out ; 

 the cells of which it is composed are considerably larger 

 than before, and the granular nature of the protoplasm is 

 still more evident. With picro-nigrosin staining these cells 

 become faintly, yellowish. The mesodermal papilla has be- 

 come more or less hollow, and constitutes a kind of pulp- 

 cavity. The whole structure is strikingly like that of a 

 developing hair. 



