66 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



Spores) 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 con- 

 ditions) into sexual flukes (Fig. ii8). In this case, 

 reproduction by special cells, like undifferentiated pre- 

 cocious 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 " 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 Amelia, 

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

 illustrate a rhythm between asexual and sexual multi- 

 plication, between parthenogenetic and normal sexual 

 reproduction, between vegetative and active life, between 

 a relatively " anabolic " and a relatively " katabolic " 

 preponderance. 



II. Embryology 



Egg-cell or ovum.^ — Apart from cases of asexual repro- 

 duction and parthenogenesis, every multicellular animal 

 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 

 essential qualities of the fertilised ovum from which the 

 parent animal was developed. 



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

 plasm is a complex colloidal mixture of substances ; its 

 nucleus or germinal vesicle contains the usual chromatin 

 elements ; it has often a store of reserve material or 

 yolk, and a distinct envelope representing a cell wall 

 (Figs. 31 and 37). 



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 

 with the middle layer or mesoderm, usually on an area of 

 the epithelium lining the body cavity. In lower animals 



