ORIGIN OF THE INDIVIDUAL 



217 



FIG. 116. Hydra reproducing aaexu- 

 ally by dividing lengthwise. (After 

 Koelitz.) 



Though Volvox and other colonial forms afford a glimpse 

 of the conditions which probably prevailed when the evolu- 

 tionary bridge from unicellu- 

 lar to multicellular organ- 

 isms was crossed, the varied 

 methods of reproduction of 

 the latter by no means in- 

 dicate the early establish- 

 ment of a hard and fast 

 boundary between soma and 

 germ. Many of the In- 

 vertebrates, such as Hydra 

 and various types of worms, 

 reproduce not only by germ cells, but also by strictly asexual 

 processes which are known as FISSION and 

 BUDDING. These processes are comparable 

 merely in a superficial way with the similarly 

 named methods in the Protista. In some 

 forms the whole complex body divides into two 

 or more parts, each of which reforms 

 REGENERATES what was lost and so becomes 

 a complete though a smaller individual. In 

 other forms, as well as in Hydra itself, buds 

 arise as outgrowths from the body and develop 

 into replicas of the parent either before or after 

 becoming detached. (Figs. 116, 117.) 



In many of the nearest allies of Hydra the 

 buds remain permanently attached so that 

 eventually a large colony of organically con- 

 nected hydra-like individuals (HYDRANTHS) is 

 formed. (Fig. 64.) This condition leads to a 

 physiological division of labor between the various hydranths 

 which may become more or less modified in structure so that, 



FIG. 117. An 

 unsegmented 

 worm (Flat- 

 worm) in pro- 

 cess of fission. 

 (After Child.) 



