GENERAL EMBRYOLOGY. 



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one another into a colony, never produce sexual organs, 

 but sexual animals, the medusa. The medusa? are alto- 

 gether unlike the polyps, being much more highly organ- 

 ized, and freely motile; only very rarely have they pre- 

 served the asexual mode of reproduction; on the other 

 hand, they develop eggs and spermatozoa, from which the 

 non-motile nurses, the polyps, develop. This example 

 shows that, in alternation of generations, there is present 



FIG. 88. Bovgahivillea rainosa. (From Lang.) /;, hydranths which have given rise to 

 medusa-buds (ink) (nurse.) ; /, separated medusa, Margelis ramosa (sexual animal). 



not only a difference in the mode of reproduction, but 

 that, usually, there obtains, in addition, a difference in 

 form and organization. Between polyp and medusa the 

 difference is so great that for a long time these two, though 

 representatives of the same species, were referred to quite 

 different classes of the animal kingdom. In many cases 

 the alternation of generations may be still further compli- 

 cated by two asexual generations following each other, 



