526 



GENERATION. 



[CHAP, xxxvi. 



Fig. 254. 



which possess the power of development, under favourable circum- 

 stances, into a form identical with that from which they were 

 produced. In consequence of this property, the bud of a plant 

 has been termed a phyton ; and a tree must, therefore, be looked 

 upon as an assemblage of these phytons. We must, however, bear 

 in mind, that all buds have not this power, as, for instance, flower 

 buds do not give rise to the formation of new buds of any sort, 

 but produce seeds. 



Amongst the lower animals, reproduction by buds is very com- 

 mon, and can be readily examined in the vorticellse and polyps. 



In the hydra, the first change 

 which is observed consists in the 

 formation of a little elevation 

 which soon becomes globular; next 

 a cavity is formed in this globular 

 mass, and becomes continuous 

 with that of the parent. After a 

 time the channel of communica- 

 tion closes, and the bud begins to 

 assume the form of a polyp, which 

 ultimately drops off; and in this 

 way a new creature is formed. 

 The echinococci multiply by the 

 formation of buds upon the in- 

 ternal surface of the hydatid vesicle. At first they are attached 

 by a sort of stem ; but ultimately they become free, and move 

 about in the fluid of the parent cyst by aid of their hooks and 

 suckers. 



A bud differs from an ovum in the important particular, that it 

 contains within itself the power of development, while the latter 

 is incapable of becoming developed into the form of its parent 

 until it has been subjected to the action of the contents of another 

 cell. The only resemblance between a bud and an ovum is, that 

 in both the organization is imperfect. 



True Generation. The processes of multiplication above referred 

 to must be distinguished from the one which we are now about 

 to consider. True generation consists in the union of the contents 

 of two different cells, called respectively the "sperm cell" and 

 " germ cell," and the production of a structure differing from both, 

 from which the new being is ultimately evolved. The simplest form 

 of this process is seen in the lower algse in conjugation. At first, 

 the opposite cells of two filaments are seen to be swollen on the 



Figures of the fresh-water hydra and vorti- 

 cella, shewing the multiplication of new indi- 

 viduals by the formation of buds. 



