IV REPRODUCTION 301 



example ol iiidividuafioi) . morphologicalh' it is equivalent 

 to an indefinite number of unicellular organisms : but, 

 these acting in concert, some taking one duty and some 

 another, form, physiologically speaking, not a colony of 

 largely independent units (compare p. 281), but a single 

 multicellular individual. 



Hydra has two distinct methods of reproduction, asexual 

 and sexual. 



Asexual multiplication takes place by a process of budding. 

 A little knob appears on the body (Fig. 73 a, hd^), and is 

 found by sections to arise from a group of ectoderm cells ; 

 soon however it takes on the character of a hollow out- 

 pushing of the wall containing a prolongation of the enteron, 

 and made up of ectoderm, mesogkea, and endoderm. (Fig. 

 74, A, bdy). In the course of a few hours this prominence 

 enlarges greatly, and near its distal end six or eight hollow 

 buds appear arranged in a whorl (Figs. 73, a, and 74, a, 

 hd'^^. These enlarge and take on the characters of ten- 

 tacles, and a mouth is formed at the distal end of the bud, 

 which thus acquires the character of a small Hydra (Fig. 73, 

 A, bd^). Finally the bud becomes constricted at its base, 

 separates from the parent, and begins an independent ex- 

 istence. Sometimes, however, several buds are produced at 

 one time, and each of these buds again before becoming 

 detached ; in this way temporary colonies are formed. But 

 the buds always separate sooner or later, although the\- 

 frequently begin to feed while still attached. 



It is a curious circumstance that Hydra can also be mul- 

 tiplied by artificial division : the experiment has been tried 

 of cutting the living animal into pieces, each of which was 

 found to grow into a perfect individual 



The sexual organs or gonads (p. 193) are of two kinds, 



