138 AN INTRODUCTION TO ZOOLOGY 



into two or four parts, becomes a perfect polyp (Fig. 64, A) 

 that when the head end is split in two and the parts separated 

 slightly a two-headed animal results; and that a specimen when 

 turned inside out is able to readjust itself to new conditions forced 

 upon it. 



Regeneration may be denned as the replacing of an entire 

 organism by a part of the same. It takes place not only in 

 Hydra, but in many other Ccelenterates, and in some of the rep- 

 resentatives of almost every phylum of the animal kingdom. 

 Hydra, however, is the species that has been most widely used 

 for experimentation. Pieces of Hydra that measure J mm. or 

 more in diameter are capable of becoming entire animals (Fig. 

 64, B). The tissues in some cases restore the lost parts by a 

 multiplication of their cells ; in other cases, they are worked over 

 directly into a new but smaller individual. 



GRAFTING. Parts of one Hydra may easily be grafted upon 

 another (Fig. 64, C). In this way many bizarre effects have been 

 produced. Parts of two Hydras of two species have also been 

 successfully united. 



GENERAL REMARKS ON REGENERATION. Space will not per- 

 mit a detailed account of the many interesting questions involved 

 in the phenomena of regeneration, but enough has been given to 

 indicate the nature of the process. The benefit to the animal of 

 the ability to regenerate lost parts is obvious to all. Such an 

 animal, in many cases, will succeed in the struggle for existence 

 under adverse conditions. The regeneration of the earthworm 

 and the crayfish are considered in Chapters X and XI. It will 

 suffice here, therefore, to say a few words concerning regenera- 

 tion in general. Regeneration takes place continually in all 

 animals; for example, new cells are produced in the epidermis of 

 man to take the place of those that are no longer able to perform 

 their proper functions. Both internal and external factors have 

 an influence upon the rate of regeneration and upon the character 

 of the new part. Temperature, food, light, gravity, and contact 

 are some of the external factors. In man, various tissues are 



