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. 

 13). 



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 tipper 

 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 

 Wood's Holl, 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 



