xiii.] 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. 



