GENERATION, SEX AND ONTOGENY 



223 



than that of the female and differing greatly from it in structure. 

 In some cases even as, for example, the worm which causes 

 "gapes " in chickens the male lives parasitically on the female, 

 being attached to her body for its whole lifetime, and draw- 

 ing its nourishment from her blood (Fig. 130). 



Some of the complex animals are hermaphroditic that is, 

 a single individual produces both egg 

 cells and sperm cells. The tapeworm 

 and many allied worms show this con- 

 dition. This is the normal condition for 

 the simplest animals, as we have already 

 learned, but it is an exceptional condition 

 among the complex animals. 



However the beginnings of the new 

 organisms are produced, whether asexu- 

 ally or bisexually (whether, that is, by 

 simple division, budding, sporulation, or 

 as true but unfertilized eggs, or as eggs 

 with a nucleus made by the fusion of two 

 germinal nuclei from male and female in- 

 dividuals respectively, or from an her- 

 maphroditic individual), this new organism 

 in embryo has a shorter or longer course 

 of development and growth to undergo, 

 before it, in turn, is in condition to pro- 

 duce new individuals of its kind. 



Certain phenomena are familiar to us 

 as recurring inevitably in the life of every 

 animal which we familiarly know. Each 

 individual is born in an immature or 

 young condition; it grows (that is, it in- 

 creases in size) and develops (that is, 

 changes more or less in structure) and dies, 

 occur in the succession of birth, growth, and development, and 

 death. But before any animal appears to us as an independent 

 individual that is, outside the body of the mother and outside 

 of an egg (i. e., before birth or hatching, as we are accustomed 

 to call such appearance) it has already undergone a longer 

 or shorter period of life. It has been a new living organism 

 hours or days or months, perhaps, before its appearance to us. 

 This period of life has been passed inside an egg, or as an egg, 



FIG. 130. The parasitic 

 worm, Syngamus trache- 

 alis, which causes the 

 gapes in fowls. The 

 male is attached to 

 the female and lives 

 as a parasite on her. 



These phenomena 



