I.] THE FROG. 157 



[If an induction-coil is not at hand a bit of clean 

 copper wire twisted round a strip of zinc, with the 

 points of contact moistened with dilute acetic acid, 

 may be used to stimulate the nerve ; smart tapping or 

 pinching with a pair of forceps will also excite it, but 

 by such means the nerve is soon killed.] 



The above experiments show : 



c. That the muscle is irritable and contractile: certain 

 external agencies (stimuli] excite some change in it, 

 the result of which is a muscular contraction. 



d. The nerve is irritable: certain external agencies ex- 

 cite some change in it, which in this particular case 

 manifests itself by a contraction of the muscles con- 

 nected with the nerve. 



e. The nerve possesses conductivity : although it is stimu- 

 lated at some distance from the muscles, the change 

 excited by the stimulus travels along it to them. 



O. Development. 



Place some freshly-deposited frog's spawn (taken di- 

 rectly after extrusion by the female) in a table aquarium 

 or other glass vessel filled with water. Examine and 

 preserve the eggs and embryos as directed below. 



It is necessary for both observation and preservation 

 that the mucus investment which surrounds the egg 

 should be removed; this may best be done with a 

 couple of fine needles, under water in a small saucer, or 

 on any convenient white surface. When liberated, 

 float the objects into a shallow watchmaker's glass, and 

 examine under a low power as opaque objects, on a 

 white ground unless otherwise directed. Preserve in 

 Kleinenberg's picric acid solution, and transfer to 

 alcohol of increasing strengths. 



