300 Miscellaneous. 



use of reagents, such as alcohol, solutions of chromic acid, picric 

 acid, &c. By these means the preparations are preserved so well 

 that, with precautions, they may be mounted in Canada balsam. 



Under suitable conditions, on nearly all the scales there will be 

 found, outside the perfect spinules, two of these organs, one on each 

 side, in course of development ; and by multiplying the observations, 

 we may form a clear idea of the origin of these parts. 



In the most rudimentary state that I have been able to observe, 

 the spinule is reduced to a sort of flattened cone, 0*03 millim. in 

 diameter at its base, and of about the same height. It is surrounded 

 by cells measuring from 0-0U9 to O014 millim., resembling the 

 other epithelial elements in their aspect and dimensions, but distin- 

 guished from them nevertheless by their arrangement in a more or 

 less spherical mass. The cone is composed of a very finely granular 

 substance, especially after the action of certain reagents, such as 

 acetic acid ; it must be regarded as intended to furnish the materials 

 necessary for the growth of the spinule — in fact as the spinular 

 papilla, the spherical mass representing a true follicle. 



A little later the follicle becomes less distinct, and even disappears 

 completely ; at the same time the other parts become more com- 

 plicated. The papilla always remains in the same state, and nearly 

 in the same form ; but its point is as it were hooded by a transparent 

 hyaline sheath, the first rudiment of the dentinous substance which 

 will form the perfectly developed organ. At this time, if the pre- 

 paration is treated with acetic acid, the tissue of the papilla be- 

 comes granular, while the sheath becomes pale ; no effervescence or 

 sensible diminution of volume is observed, which must be attributed 

 to the small quantity of calcareous matter which the tissue then 

 contains compared with the abundance of the organic part. 



The development is continued to the complete state by the 

 gradual augmentation of the dentinous cone. One of the first effects 

 is to hide the papilla ; then the point lengthens, and ends by piercing 

 the epidermic festoon. 



The papilla seems to persist even in the perfectly developed 

 spinules. On treating the scale with a strong acid (such as hydro- 

 chloric or nitric acid) to get rid of the calcareous salts, we see the 

 aspect of the spinules become greatly modified. The length becomes 

 scarcely half that of the entire organ ; the granular substance con- 

 stituting the papilla appears in the form of a sort of basal bud, 

 produced, towards the free margin of the epidermic mass, by a 

 cylinder formed of the same substance. In the young spinules the 

 papilla alone remains ; in the perfect spinules the cylindrical pro- 

 longation is hooded by the organic substance of the decalcified 

 dentine, persisting under the appearance of a transparent, elastic, 

 hyaline layer, in which a sort of fibrous longitudinal structure may 

 be made out. Under the action of the reagent, and in consequence 

 of the disengagement of carbonic acid, this gas is very often accu- 

 mulated under the epidermis, between the lamella and the row of 

 spinules, and separates them, putting the discontinuity of these 

 parts beyond a doubt. 



