408 TEXTBOOK OF ZOOLOGY 



In most of the single-celled organisms reproduction may occur 

 by such equal division of the protoplasm (binary fission) that the 

 new individuals cannot be distinguished as parent and offspring. 

 Protozoa may reproduce also by sporulation, by which process the 

 cell forms a protective cyst and by a series of simple divisions (frag- 

 mentation) the internal protoplasm breaks into a number of smaller 

 units. Following this the cyst ruptures and releases these new units 

 as independent individuals. For the most part, reproduction among 

 protozoans is taken to be asexual, but according to a recently pub- 

 lished work by Sonneborn, a distinct sexuality exists in Paramecium. 

 Examples of asexual reproduction by budding and fission have al- 

 ready been pointed out in the studies of reproduction of sponges, 

 hydra, planaria, and even in tunicates. 



Sexual Reproduction. — In certain of the colonial Protozoa, volvox 

 for example, the colony may reproduce for several generations by 

 asexual division of the individual cells but sooner or later the cells 

 of the colony become specialized into conjugating individuals. In 

 some forms this goes to the extent of certain cells becoming distinct 

 gametes with male and female characteristics. In such forms it is 

 possible to see foreshadowed sexual reproduction as it is known in 

 Metazoa. 



In the simplest of Metazoa, as in sponges, there are no specially 

 organized gonads for the production of germ cells, but as a rule the 

 germ cells are produced in such organs set apart for this purpose. 

 The ovary produces mature or nearly mature ova and the testis pro- 

 duces mature spermatozoa. 



Hermaphroditism is the condition in which the same individual 

 produces both ova and spermatozoa. It occurs principally in the 

 simpler Metazoa, a few higher ones, and rarely among normal verte- 

 brates. Previous studies made on the reproduction of hydra have 

 brought out that the gonads are temporary, both being formed by 

 aggregations of formative or interstitial cells between the ectoderm 

 and endoderm. After the seasonal production of germ cells is com- 

 pleted, the gonads disappear. In flatworms and annelid worms the 

 gonads are permanent structures of the mesoderm. Both ovaries and 

 testes are present. Even in these true hermaphrodites cross-fertiliza- 

 tion is insured by copulation or union in such a way that the sper- 

 matozoa of one individual fertilize the ova of another. In certain 



