xin.] THE FROG. 265 



appear as granular spherical masses with a clearer 

 central patch. 



b. Examine with a high power a portion of your 

 specimen containing some of the younger and 

 more transparent ova. Note 



a. The thin structureless membrane, vitelline 

 membrane, enveloping each. 



/?. The granular matter (yelk, vitellus] forming 

 most of the ovum. It sometimes appears to 

 be composed of an outer granular and an 

 inner clearer layer. 



y. The clearer central mass (germinal vesicle) im- 

 bedded 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 

 unconscious, which will probably be in a few se- 

 conds. 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 ver- 

 tebral column. By this process (known as pithing) 

 the frog is rendered totally incapable of further con- 

 sciousness, though most of its tissues will retain their 

 vitality for some time. 



