A L TERN A TJON OF GENERA TIONS. 53 



usually produced, that they have some of the qualities of 

 male-cells. We may think of them as reversions to the 

 primitive type of germ-cell, such as was probably produced 

 by Metazoa in which neither maleness nor femaleness was 

 as yet differentiated. 



(c) Alternation of Generations. — A fixed asexual hydroid 

 or zoophyte (campanularian or tubularian) often buds off 

 and liberates sexual medusoids or swimming-bells, whose 

 fertilised ova develop into embryos which become fixed and 

 grow into hydroids. This is the simplest illustration of 

 alternation of generations. 



The liver-fluke {Distomuni hepaticum) of the sheep pro- 

 duces eggs which when fertilised grow into embryos. 

 Within the latter, certain cells (which can hardly be called 

 eggs) grow into numerous other larvae of a different form. 

 Within these the same process is repeated, and finally, 

 the larvae thus produced grow (in certain conditions) into 

 sexual liver-flukes. In this case, reproduction by special 

 cells like undifferentiated precocious ova, alternates with 

 reproduction by ordinary fertilised egg-cells. So, too, the 

 vegetative sexless " fern-plant " gives rise to special spore- 

 cells, which develop into an inconspicuous bisexual " pro- 

 thallus," from the fertilised egg-cell of which a " fern-plant " 

 springs. 



Various kinds of alternation are seen in the life-cycle of 

 the fresh-water sponge, in the stages of the jellyfish Aurelia, 

 in the history of some " worms " and Tunicates. They 

 illustrate a rhythm between asexual and sexual multiplica- 

 tion, between parthenogenetic and normally sexual reproduc- 

 tion, between vegetative and animal life, between a relatively 

 " anabolic " and a relatively " katabolic " preponderance. 



II. Embryology. 



T/ie Egg-cell or Ovum. — Almost every multicellular 

 organism begins life as an egg-cell with which a male-cell or 

 spermatozoon has entered into intimate union. The excep- 

 tional occurrence of asexual multiplication and parthenogen- 

 esis has been explained. 



The most important characteristic of the reproductive cells. 



