100 ELEMENTARY ZOOLOGY. 



Similarly, some animals, much more complex than 

 these single-celled forms, show the same process of divi- 

 sion or budding. Reference has already been made to 

 Hydra and the worms (94). This is a rather common 

 process in the lower groups of animals. In such cases 

 each of the offspring is a half or some large part of the 

 whole animal, and has the power of producing a perfect 

 new animal of the same kind. 



It is not difficult to see that such a plan as this would 

 be more and more difficult to carry out, as animals become 

 more complex and highly organized. Because of this 

 fact we could scarcely conceive of the grasshopper or 

 the sparrow reproducing as the Hydra does in Fig. 46. 

 Besides, it is a very expensive performance a great drain 

 on the parent; and nature is economical wherever she 

 can be with advantage. 



For the complex animals a more economical device has 

 been found, thus: the adult, instead of putting aside as a 

 daughter individual a large portion of its body, separates 

 one or more single cells, each of which has the power, 

 under favorable conditions, of developing into the adult 

 of its species. All the possibilities of the parent are 

 stored in this cell. By this device many offspring can 

 be produced as economically as a few by the earlier 

 methods. In this way provision is made for the high 

 death rate among the young, because of their enemies 

 and from other unfavorable conditions of life. 



1 06. Males and Females in Reproduction. The dif- 

 ferences between adult males and females are continued 

 into their reproductive cells. When females reproduce 

 they do so by rather well-nourished, rounded, sluggish 

 cells known as ova (Fig. 8, 3) . Males reproduce by means 



