56 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



defined as the alternate occurrence in one life cycle of two 

 (or more) different forms differently produced. 



The liver fluke {Distoma hepaticum) of the sheep produces 

 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 flukes 

 (Fig. 54, p. 1 68). In this case, reproduction by special cells 

 Uke undifferentiated precocious ova, alternates with reproduc- 

 tion by ordinary fertilised egg cells. So, too, the vegetative 

 sexless " fern plant " gives rise to special spore cells, which 

 develop into an inconspicuous bisexual " prothallus," from 

 the fertilised egg cell of which a " fern plant " springs. 



Various kinds of alternation are seen in the life C3'cle of 

 the fresh water sponge, in the stages of the jelly fish Anrelia, 

 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. 



The Egg Cell or Ovum. — Apart from cases of asexual re- 

 production and parthenogenesis every multicellular organism 

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

 tozoon has entered into intimate union. 



The most important characteristic of the reproductive 

 cells, whether male or female, is that they retain the essen- 

 tial qualities of the fertilised ovum from which the parent 

 animal was developed. 



The ovum has the usual characters of a cell ; its sub- 

 stance is traversed by a fine protoplasmic network ; its 

 nucleus or germinal vesicle contains the usual chromatin 

 elements ; it has often a distinct sheath representing a cell 

 wall. 



In Sponges, the ova are well nourished cells in the middle 

 stratum of the body ; in Coilentera they seem to arise in 

 connection with either outer or inner layer (ectoderm or 

 endoderm) ; in all other animals, they arise in connection 



