66 AH INTRODUCTION TO ZOOLOGY. 



the algae, the Conjugatce and Zoosporece.^ The cell, so 

 formed by union of two cells, may now become en- 

 cysted; that is to say, the soft protoplasm forms a 

 hard wall round itself, and becomes round and motion- 

 less for a long time. This stage of encystment, again, 

 is very like a form assumed by tbe cells resulting 

 from conjugation ia certain vegetable cells,^ which 

 get a special hard coat, and remain dormant for a 

 long time, and are tlius enabled to resist the results 

 of dry weather or other accident ; resting cells of this 

 kind are called resting spores, spore being the name 

 given to the reproductive cells of the lower vegetable 

 forms. After a time the encysted form again be- 

 comes active, breaks through, its hard shell or cyst, 

 and leaves it behind. It will next divide into two or 

 more new cells. We have here the simplest type of 

 sexual reproduction. In Vorticella, the two cells that 

 unite are slightly difEerenb from one another; one is 

 larger, and is borne on a stalk ; the other is with- 

 out a stalk, and swims freely. Here we have an 

 indication of the differences between male and 

 female, the free and more active cell representing 

 the male. 



In multicellular animals, as we have already said, 

 there are special reproductive cells, contained in 

 special reproductive organs of a more or less com- 



' See Prantl acd Tines' " Text-book of Botany," chapter on 

 the Thallophyta. 

 " Zoospores. 



