HISTOLOGY 39 



transfer them to the water to wash off the surplus stain; mount 

 in a clean drop of water. If the preparation is successful, the nuclei 

 will be stained a deeper purple than the remainder of the fiber. Use 

 these two slides in the following studies. For nuclei, the stained ma- 

 terial should be used; for other features, the unstained fibers will be 

 more satisfactory. 



(o) Each muscle fiber is a single cell with a large number of 

 nuclei. Compare the length of such a cell with other cells you have 

 seen. Each fiber is composed of a large number of very minute 

 fibrillae which are indicated by rather indistinct longitudinal mark- 

 ings. There is a very much more distinct transverse striation, the 

 feature from which this kind of muscle derives its name. Each fiber 

 is enclosed in a very delicate cell membrane, the sarcolemma, which 

 cannot be recognized except at places where the fiber is somewhat 

 broken. Measure the width of a single cell as projected to table 

 level. Make a drawing of this cell (3 X its measured size). Record 

 the total magnification and the source of the tissue. Review and cor- 

 relate your observations on the examples of tissues by means of the 

 table in Fig. 27. 



C. Organs 

 Exercise 37.— The Skin. 



(a) The skin of the frog will be studied in permanently stained 

 and mounted sections cut at right angles to the surface. These sec- 

 tions have been stained with two different dyes so that it is easy 

 to distinguish the cytosome and nucleus of a cell. The skin is made 

 up of two layers of which the outer is the epidermis and the inner is 

 the dermis. The epidermis is composed of stratified squamous epi- 

 thelium. Its flattened outermost cells have been studied from surface 

 view in Exercise 28. What is the shape of the cells in the deeper 

 layers? The dermis is composed of fibrous connective tissue in which 

 numerous blood vessels are found. A layer of pigment cells will be 

 observed immediately below the epidermis and another one near the 

 base of the dermis. Pigment granules also occur in some of the epi- 

 dermal cells. Notice the large, simple alveolar glands which open 

 to the exterior by ducts. To which of the two layers of the skin do 

 the cells lining the ducts and glands belong? What is the function of 

 these glands? Have you anything comparable in your own skin? 

 Can you identify non-striated muscle cells surrounding the expanded 

 part of the gland? What is their function? Make a drawing (2 X 

 the measured size projected at table level) of a narrow vertical strip 

 through the skin as seen with the high-power objective, showing in 



