254 LECTURES ON BIOLOGY 



Among the vertebrates the tailed amphibians possess the 

 most highly developed regenerative power. Axolotls and Tritons 

 are not only able to regenerate their tails but also their legs. 

 That lizards are able to regenerate a new tail we have already 

 heard, but they are not able to regenerate the legs. Among 

 fishes, birds, and mammals it is no longer possible to speak of 

 regeneration, for with them re-growth is restricted to restoring 

 the skin and healing slight wounds. We perceive, therefore, that 

 the faculty of regeneration decreases in proportion as the differ- 

 entiation of the animal body increases. The fact that embryos, 

 including those of the highest vertebrates possess, generally 

 speaking, a considerable regenerating power well accords, there- 

 fore, with our hypothesis concerning the causes of regeneration. 



We have so far seen that regeneration of different body-parts 

 appears usually as the result of pathological changes. But it 

 may also be observed in the normal course of life of numerous 

 species of animals, when it is closely associated, as I have already 

 mentioned, with asexual reproduction. As among the protozoans 

 individuals sometimes ' grow beyond their individual measure,' 

 and then divide into two or more individuals, a similar process 

 may be observed in the higher organisms. But as the sections 

 are naturally deficient in certain parts and organs, these must be 

 supplemented either before or after constriction. The more 

 frequent and more practical course, however, is when division 

 as in Infusorians is preceded by regeneration, and the most 

 important organs are at a certain time ' laid down ' in several 

 sets. Only when that has been done separation takes place. 

 The young in that case are like their mother in every detail, 

 excepting alone in size, 



This method of reproduction may be very distinctly observed 

 in a little Turbellarian, the Microstoma, When it has reached a 

 certain size a little circular sulcus becomes visible in the centre 

 of its body. Below this dividing line there appear a new 

 mouth-aperture, primary optic spots, and other important organs 

 of the region of the head. The animal is now ready to divide 

 into two individuals. In Microstoma the asexual reproduction 

 frequently proceeds so rapidly that new dividing lines may be 

 observed in both halves long before the constriction has become 

 complete, and this generation of grandchildren sometimes bears 



