6 4 MANUAL OF ANIMAL BIOLOGY 



the bud are continuous with the corresponding parts of the parent 

 Hydra. A section through it shows that the body wall is made 

 up of ectoderm, mesogloea, and endoderm surrounding a central 

 enteric cavity, just as is the body wall of the parent Hydra. When 

 the bud is fully developed the attachment with the parent Hydra 

 is broken, a basal disc is developed, and the new individual begins 

 an independent existence. (W. f. 57.) 



In addition to asexual reproduction by budding it is also known 

 that Hydra may reproduce by fission — that is, by a division of 

 the entire animal into two individuals. The division is generally 

 along the longitudinal axis, beginning at the distal, or tentacle, end 

 of the animal and proceeding toward the foot. (W. f. 155.) 



In connection with the asexual reproduction of Hydra, consid- 

 eration should be given to the remarkable power of regeneration 

 which this animal possesses. Regeneration may be defined as the 

 ability of an organism to replace, or regenerate, missing parts in 

 order to restore again the perfect whole. The power to regenerate 

 lost parts varies greatly in different kinds of animals, but, in gen- 

 eral, the ability exists in inverse ratio to the amount of cell spe- 

 cialization which is present. In other words, the higher the devel- 

 opment of an animal, the less is its power of regeneration. Hydra 

 is a comparatively simple type of Metazoon, and experiments on 

 this organism have shown that it can be cut into several pieces — 

 four or more and in many different planes, and each of these 

 pieces will regenerate the missing parts in a short time. Thus, 

 as a result of sectioning the animal and the power of regeneration 

 which the tissues possess, there will be several individuals where 

 previously there was only one. Regeneration in such cases really 

 results in the asexual reproduction of the Hydra. (W. f. 158.) 



Sexual Reproduction. Under certain conditions which are 

 not known, some species of Hydra develop male and female 

 gonads, both of which are temporary structures. Thus, the same 

 individual may produce both sperm and eggs. In the higher ani- 

 mals, a single individual produces either sperm or eggs. An animal 

 which produces both sperm and eggs is known as an hermaphro- 

 dite, as previously noted. (W. f. 154. C.) 



The male gonads, or testes, in which the sperm are formed, 

 develop as swellings in the ectoderm of the body wall, just below 

 the tentacles. The testis consists of an outer covering of ectoderm 

 cells which encloses a great number of the developing sperm cells. 



