124 THE FROG CHAP. 



4. Pavement or Squamous Epithelium. Take a bit of frog's 

 skin which has been kept for a day or two in Ranvier's 

 alcohol, scrape the outer surface with a sharp knife, tease 

 out and examine the edges of the scrapings in a drop of 

 water, afterwards staining with magenta (Fig. 30). 



Note the flattened cells fitting together like tiles in a pave- 

 ment, each one with its nucleus. Sketch. 



5. Unstriped Muscle. Snip off a small piece from an 

 inflated urinary bladder of a frog which has been preserved 

 in formaline, wash with water, and mount. Or, snip off a 

 very small piece not bigger than a pin's head from the 

 muscular coat of the intestine or stomach (or from a urinary 

 bladder) which has been in Ranvier's alcohol for at least 

 twenty-four hours ; then tease out in a drop of water very 

 thoroughly. Note the elongated unstriped muscular fibres 

 tapering at both ends, each containing a nucleus (Fig. 31). 

 Stain with magenta. Sketch. 



6. Striped Muscle. Snip off a small piece about Jth inch 

 long from any of the body-muscles of a freshly-killed frog, 

 put it on a slide in a drop of salt solution, and tease it 

 out with the grain, i.e., in the direction of the length of the 

 fibres. The fibres will readily separate from one another : 

 the teasing process must be stopped as soon as they are 

 apart, and care must be taken not to tear or crush the 

 individual fibres, which are large enough to be readily dis- 

 tinguishable with a magnify ing-glass. 



Observe under the low power of the microscope the long 

 cylindrical fibres (Fig. 32, A), bound together by connective- 

 tissue, and showing a distinct transverse striation and a less 

 distinct longitudinal striation. Examine a single fibre under 

 the high power (Fig. 32, B), and make out the sarcolemma 

 and the numerous nuclei, which will be rendered more 

 distinct by the addition of acetic acid or by staining. Sketch. 



7. Connective-tissue. Carefully separate two of the 

 muscles of the leg in a fresh frog, and note the delicate web 

 of connective-tissue between them : or, note the fine strands 

 of connective-tissue between the skin and the muscles of the 

 body-wall. With fine forceps lift up a small shred of this, 

 snip it off with scissors, and place it on a dry slide. Then, 

 with two needles, spread it out on a thin, even layer, breath- 

 ing on it occasionally to prevent drying. Lastly, place a 

 drop of salt solution on a cover-glass and quickly lower it 

 on the preparation. The reason for this procedure is that 

 if connective-tissue is placed in fluid, it contracts into a little 

 lump, which is too opaque for examination and cannot be 

 readily spread out. 



