HISTOLOGICAL DEMONSTRATIONS. 67 



Apply a drop of Farrants' solution or glycerine ; cover, and 

 examine. 



(L.) The epidermis consisting of squamous cells 

 roughly divided into two layers, the horny layer above and 

 the rete mucosum below. The cutis vera with its papillae. 

 Select an extremely thin piece of the rete mucosum close 

 to the cutis vera ; place it exactly in the centre of the field, 

 and fix the slide in that position with clips. 



(H.) The epidermic cells are produced by fission in the 

 lowest layer of the rete mucosum. Observe the small size 

 of the cells there, as compared with the upper part of the 

 rete, where they are growing but not dividing. The margins 

 of all the cells are jagged. This can be clearly seen in the 

 rete mucosum, and in the lower part of the horny layer. 

 Towards the upper part of the horny layer the cells become 

 scaly, and the nucleus often disappears. In the lowest 

 layers the cells appear to be nucleated protoplasts. As 

 they grow old they become horny, owing to the production 

 of keratin, a substance into which the protoplasm is 

 probably transformed. 



The history of the epithelium lining the mouth is 

 similar to that of the epidermis. As it is more difficult to 

 see the jagged margins of the cells in the former, the latter 

 is taken here. (This preparation will be required again 

 when we study skin?) 



SJA.. The evidence of the fission of the cells in the deepest layers of the 

 epidermis can only be clearly seen with a power of about 800 diam. 

 The nuclei of the cells are generally elongated, and mostly contain 

 more than one nucleolus. Fission of the nucleus may sometimes be 

 seen. It cannot be doubted that the perinuclear plasm also divides, but 

 it is astonishingly difficult to see a cell in the act of division, probably 

 because they are so closely packed together. 



876. The serrated margins of the cells are well seen in skin of human 

 finger, treated by a method which we owe to Ranvier. Inject per 

 cent osmic acid ( 281) into the lower part of the epidermis by thrusting 

 the nozzle of Wood's syringe ( 344) obliquely into the cutis vera, and 

 driving the fluid wherever it will go. Harden in rectified spirit, 

 make V. S., and preserve in glycerine. The lowest epidermic cells are 

 slightly darkened, and a power of about 800 diam. shows that the 

 so-called "prickles" are not the teeth of neighbouring cells "dove- 

 tailed one with another," but that, as Dr. Martyn (Monthly Microscop. 

 "Journ., xiv., p. 59) has shown they are bridges between the cells 



