256 ELEMENTARY BIOLOGY. [XIIL 



7. The clearer central mass {germinal vesicle) 

 imbedded in the vitellus. The large number 

 of highly refracting masses (germinal spots) 

 ■within the germinal vesicle. 



K. The physiological properties of muscle and nerve. 



Place a frog under a beaker, with a drop or two of 

 chloroform : take it out immediately it becomes uncon- 

 scious, which will probably be in a few seconds. Now 

 feel with a finger-nail for the depression beneath the 

 skin at the back of the animal's head, which indicates 

 the point of articulation of skull and spinal column : it 

 lies in a line joining the posterior borders of the two 

 tympanic membranes. Divide the skin and muscles at 

 this point until the neural canal is laid open, and then 

 pass a stout wire into the cranium and down the neural 

 canal of the vertebral column. By this process (known 

 as pithing) the frog is rendered totally incapable of 

 further consciousness, though most of its tissues will 

 retain their vitality for some time. 



a. Kemove the skin from one leg, so as to lay bare 

 the muscles : send an interrupted electric current 

 through any one of them (or tap the muscle 

 sharply with the back of a scalpel) : it will 

 immediately contract, or alter its form in a 

 definite way ; it, becomes shorter and thicker, and 

 in so doing moves the bones to which it is 

 attached. 



b. Very carefully lay bare the sciatic nerve, taking 

 care not to crush or drag it : divide it as high up 

 as possible and, seizing it with a pair of forceps 

 close to its cut end, lay it over the electrodes of 

 an induction-coil, Probably when the nerve is 



