252 LECTUEES ON BIOLOGY 



consisting of one and more cells. Of a division of their bodies 

 into sharply-defined sections head, thorax, abdomen, and ex- 

 tremities so characteristic of insects and vertebrates, there is as 

 yet no vestige. The single body-parts are similar to each other, 

 the whole being not so much sub-ordinate as co-ordinate. In 

 addition, the differentiation and specialization of the individual 

 cells has not made much progress : they have rather retained 

 their embryonic character. Thus it is that injuries or mutilations 

 are easily sustained by these animals, while more highly differ- 

 entiated forms would perish from them. A Gothic dome, a Doric 

 temple, partially destroyed, is a useless ruin, no longer capable of 

 serving its purpose; but a simple beam we may, without dis- 

 advantage, cut up into several parts, and each section, though 

 smaller than the whole, will still retain the qualities of the beam. 

 The same holds true in the case of the higher, differentiated 

 animals, as compared with the lower, more homogeneous forms. 

 A vertebrate or an insect that has been cut into two parts is no 

 longer able to live. Both halves are too closely correlated : the 

 life-functions of one half are conditioned and supplemented by 

 those of the other, and they can only fulfil their numerous tasks 

 when joined in a close union, but are doomed to perish when 

 separated. But an animal whose body has not yet reached such 

 a high state of differentiation and whose structure is as yet more 

 uniform retains its power to live even after mutilation, and 

 is even able quickly to replace the lost parts by regeneration. 

 Hence numerous lower multicellular species still retain the 

 asexual form of reproduction by fission or budding, in addition 

 to reproduction by germ-cells, while representatives of the 

 highest classes of the animal kingdom are exclusively dependent 

 upon sexual preservation of their species. 



Fission, as well as bud-formation, is closely, associated with 

 the power of regeneration, for in order to retain life the animals 

 that have been cut into parts must of course be able to replace 

 the lost parts. For this reason it is clear that this faculty is far 

 greater in the lower organisms. 



The celebrated experiments made in 1740 by Trembley with 

 the green freshwater polyp Hydra viridis proved that even the 

 smallest particle of the body of the polyp is able to develop into a 

 complete organism. Jager was further able to observe in Hydra 



