204 PRACTICAL ZOOLOGY 



m.l) and escape into the surrounding water. The eggs arise in 

 the ovary (Fig. 119, y.e) and usually only one egg develops in 

 a single ovary. 



The egg (Fig. 119, m.e) is fertilized by a spermatozoon; it 

 then divides into a number of cells and is known as an embryo 

 (Fig. 119, bl ; g). The embryo separates from the parent and 

 falls to the bottom of the pond, where it remains unchanged for 

 several weeks. At the end of this time the eggshell breaks 

 away and the embryo escapes. A circlet of tentacles arises 

 at one end; a mouth appears in their midst; and the young 

 Hydra thus formed soon grows into the adult condition. 



Regeneration. — The power of animals to restore lost parts 

 was first discovered in Hydra by Trembley in 1744. This in- 

 vestigator found that if Hydras were cut into two, three, or 

 four pieces, each part would grow into an entire animal. 



Regeneration may be defined as the replacing of an entire 

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

 Hydra, but in some of the representatives of almost every 

 phylum of the animal kingdom. Hydra, however, is a species 

 that has been quite widely used for experimentation. Pieces 

 of Hydra that measure I, mm. or more in diameter are capable 

 of becoming entire animals. 



The benefit to the animal of the ability to regenerate lost 

 parts is obvious. Such an animal, in many cases, will succeed 

 in the struggle for existence under adverse conditions, since it 

 is able to regain its normal condition even after severe injuries. 

 Physiological 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. In man, various tissues are capable of re- 

 generation; for example, the skin, muscles, nerves, blood vessels, 

 and bones. Lost parts are not restored in man because the 

 growing tissues do not coordinate properly. 



Division of Labor among Individuals of a Colony. — When- 

 ever animals live together in colonies, there is almost certain to 



