APPENDIX. 
409 
frog. This nerve may be found by removing the skin from the 
back of a frog’s thigh and carefully separating the underlying 
muscles. Among them will be seen the sciatic nerve, covered 
in places with dark gray or black pigment spots. Remove a 
quarter to a half inch of the nerve, being careful to stretch it as 
little as possible ; lay it on the glass slide in a few drops of .75 
per cent, salt solution ; cautiously tear it to pieces in the direc¬ 
tion of its length with dissecting needles; then put on a cover- 
glass and examine under a high power. The nerve will be found 
to consist of a number of nerve fibres, some of which will show 
the primitive sheath, medullary sheath, and axis cylinder (Fig. 
l3) - 
The relation between the stimulation of a nerve and the con¬ 
traction of the muscle to which the nerve runs may be shown 
as follows: Expose the sciatic nerve as directed above ; then 
with the quick stroke of a sharp scalpel sever the upper end of 
the nerve as near the body as possible. At the moment of do¬ 
ing this the muscles of the leg and foot will probably contract. 
Allow the nerve to rest for a few minutes; then pinch its upper 
end with a pair of forceps. Again the muscles will contract. 
The stimulation may be repeated at intervals if the nerve.be 
allowed to rest for a few minutes between successive stimula¬ 
tions. Try also the effect of touching the nerve with a hot 
wire and with a drop of dilute acid or alkali. 
CHAPTER XX. 
The structure of the egg may be studied in the Starfish or 
Sea-urchin, Frog or Fowl. Starfish eggs preserved in various 
stages of segmentation may be purchased from the Department 
of Laboratory Supply of the Marine Biological Laboratory at 
AVood’s ILoll, Mass. Frogs’ eggs may be found in ponds and 
ditches in early spring. If transferred to the laboratory and 
kept supplied with fresh water they may be watched through 
their various stages of segmentation to the formation of the 
tadpole, its liberation from the egg, and its later development. 
Compare with Fig. 174. To watch the development of a chick, 
eggs may be incubated by a hen or in an artificial incubator, 
one egg being removed each day, and opened by breaking away 
