THE CONTINUITY OF LIFE 



233 



conditions which probably prevailed when the evolutionary bridge 

 from unicellular to multicellular organisms was crossed, the varied 

 methods of reproduction of the lat- 

 ter by no means indicate the early 

 establishment of a hard and fast 

 boundary between soma and germ. 

 Many of the Invertebrates, such as 

 Hydra and various types of worms, 

 reproduce not only sexually by eggs 

 and sperm, but also by strictly 

 asexual processes which are known 

 as fission and budding. These 



processes are comparable merely in . FlG - la ?; ~ Hy ^ ra . re P™duc- 



r • ., , m S asexually by fission. (From 



a superficial way with the similarly Koelitz.) 



named methods in the Protozoa. 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 species, 



Fig. 156. 

 A Flatworm, 



Planaria, in A y B C 



the process of Fig. 157. — Reproduction of a Flatworm, Linens socialis, 



fission. (From by fission. A, mature worm; B, same, in nine parts: C, regen- 



Child.) eration of each resulting in normal smaller worms. (From Coe.) 



as well as in Hydra itself, buds arise as outgrowths from the body 

 and assume the form of the parent either before or after becoming 

 detached. (Figs. 155-157.) 



