58 BIOLOGY OF DEATH 



ating action on tiie spermatozoon and (2) an activating 

 action on the egg. In other words, the spermatozoon is 

 conceived, by means of a substance which it bears and 

 which enters into imion with the f ertilizin of the egg, to 

 release the activity of this substance within the egg." 

 From the standpoint of the present discussion it is ob- 

 vious that Lillie's results so far present nothing which 

 in any way disturbs the conclusion we have reached as 

 to the essentially physico-chemical nature of the processes 

 which condition the continuation of life and development 

 of the egg. 



TISSUE CULTUEE IN VITBO 



Let us turn now to another question. Are the germ- 

 cells the only cells of the metazoan body which possess 

 the characteristic of potential immortality? There is 

 now an abundance of evidence that such is not the case, 

 but that, on the contrary, there are a number of cells and 

 tissues of the body, which, under appropriate conditions, 

 may continue living indefinitely, except for the purely 

 accidental intervention of lethal circumstances. Every 

 child knows that all the tissues do not die at the same time. 

 It is proverbial that the tail of the snake, whose head and 

 body have been battered and crushed until even the small 

 boy is willing to admit that the job of killing is complete, 

 ' ' will not die until the sun goes down. ' ' Galvani 's famous 

 experiment with the frog's legs only succeeded because 

 some parts survive after the death of the organism as 

 a whole. As Harrison points out "Almost the whole of 

 our knowledge of muscle-nerve physiology, and much of 

 that of the action of the heart, is based upon experiments 

 with surviving organs ; and in surgery, where we have to 

 do with changes involved in the repair of injured parts, 



