Development and Growth of the Epidermis. 89 



secondary cells measuring tjW&i otners measuring jqVo 10 > 

 and in the interstices, primitive granules, aggregated granules, 

 and nucleated cells. 



My observations, it will be seen, have been chiefly directed 

 to the epidermis, and I am prevented at present from carry- 

 ing them further, but I have no doubt that the epithelium will 

 be found to be identical in the phaenomena of development 

 and growth with the epidermis. I have observed the same 

 structure in the epithelium of the mouth and fauces, and also 

 in that of the bladder and vagina. Incomplete epithelial cells, 

 measuring 7 jyth and y^th of an inch from the fauces, pre- 

 sented a very remarkable appearance; they had a rounded 

 lobulated border, evidently composed of a row of secondary 

 cells and a depressed centre, as though the action were sub- 

 siding in the latter, while it was progressing in the circumfe- 

 rence. 



Another illustration of the structure now described, I found 

 in the cells of melanosis, and in the pigmentary cells of the 

 choroid membrane of the eyeball. I am induced to believe 

 that the same structure will be discovered more extensively 

 than at present can be anticipated. The corpuscles of mela- 

 nosis, according to my observations, are parent-cells, having 

 an average admeasurement of T o\jo tn °* an mc ^> containing 

 secondary cells and nucleated and aggregated granules, as 

 well as separate primitive granules. The " aggregated gra- 

 nules" measured from TTtJoo tn to ^oVo^ 1 °f an inch, and the 

 primitive granules about 2onoU tn - 



There is another feature in the history of development of 

 the epidermic cell, which I regard as peculiarly interesting. 

 This relates to an organic change taking place in the assimi- 

 lative powers of the primitive granules, by which the latter 

 are altered in their colour, in short, are converted into "pig- 

 ment granules." Pigment granules appear to differ in no re- 

 spect from the " primitive granules," excepting in that of co- 

 lour, and perhaps also in chemical composition. They have 

 the same globular form, the same size, and occupy the same 

 position in the cell, being always accumulated around the nu- 

 cleus, and dispersed less numerously through the rest of the 

 cell. The nucleus of the cell in the epidermis of the Negro 

 appears to consist wholly of pigment granules, while in the 

 European there is a greater or less admixture of coloured and 

 uncoloured granules. The central granules are generally 

 lighter in tint than the rest, and give the idea of a colourless 

 nucleolus, while those around the circumference are deeper 

 coloured. Besides a difference in the depth of colour of the 

 separate granules entering into the composition of a single 



Phil. Mag. S. 3, Vol. 28. No. 185. Feb. 1846. H 



