THE DOCTRINE OF IMMORTALITY 45 



ourselves with the consideration of the phenomenon of death. 

 At present we shall devote our attention to the propagating 

 of cells. 



The kind of propagation which we find in the plant is 

 called sexual and occurs also in animals. It is, however, by 

 no means necessary that the propagation should occur by 

 sexual means. Of the methods which nature applies for the 

 multiplication of living individuals, I should like to mention a 

 few to you briefly. 



Many methods of asexual reproduction are known to us. 

 The art of increasing plants in this way is practiced by every 

 gardener, and nature also makes use of the possibilities. 

 Among animals we often find a multiplication of individuals 

 effected by simple division. The zoologist describes to us 

 the column-like growth of certain jelly-fish and the following 

 transverse division of the column, so that a number of discs 

 arise, each of which becomes a jelly-fish. Asexual reproduc- 

 tion occurs among invertebrates in various forms. The pecu- 

 liar division of certain annelids has been already mentioned. 

 The budding zone is formed, and produces a new head and a 

 new tail. In a parasitic tapeworm we have discovered a 

 vesicular stage in the life cycle. At certain spots upon the 

 wall of the vesicle arise new heads, each of which initiates the 

 formation of a new tapeworm. Specially interesting are the 

 cases of precocious division, which we have learned about 

 recently, and in which we encounter the division of an egg 

 before the embryo proper has developed. Thus Kleinenberg 16 

 observed in certain earthworms that two individuals develop 

 regularly from one egg, an observation which has been con- 

 firmed by the American investigator, E. B. Wilson. 17 Still 

 more remarkable are the occurrences in certain parasitic 

 hymenoptera, in which not merely two but many individuals 



