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, and examine the scrapings in a drop of 

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



Note the flattened cells fitting together like tiles in a pavement, 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, and 

 wash with water. 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, and 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 distin- 

 guishable with a magnifying-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 methyl-green or acetic acid. Sketch. 



7- Counective-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 into a thin, even layer, 

 breathing 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, 



