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 

 like 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 cycle of 

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



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 Coelentera they seem to arise in 

 connection with either outer or inner layer (ectoderm or 

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



