1 86 BIOLOGICAL LECTURES. 



animal solely for the purpose of replacing lost parts, and since 

 in some animals the same part could be replaced time after 

 time, Bonnet assumed that on each occasion a new set of germs 

 was awakened. He pointed out that, since some animals are 

 more subject to injuries than others, they are supplied with as 

 many sets of germs as the times the animal is liable to be injured 

 during its natural life. 



Bonnet seems to have been especially impressed by the fact 

 that frorh the same region of Lumbriculus a head or a tail may 

 arise according to whether that region happens to lie at the 

 anterior or posterior end of the cut surface. For instance, if 

 the worm is cut into two pieces, a new tail will develop from the 

 posterior end of the anterior piece, and a new head from the 

 anterior end of the posterior piece. If, however, the cut had 

 been made a little further in front of or behind this level, the 

 same result would have followed ; hence it is clear that at every 

 level a head or a tail may develop. Which of these develops is 

 determined by the position of the region, i.e., whether it lies 

 at the exposed anterior or posterior end of a piece. Bonnet 

 interpreted this to mean that there are throughout the worm 

 head germs and tail germs. He saw that it is necessary to give 

 some further explanation of why the one rather than the other 

 kind of germ is aroused to activity, and made, therefore, a 

 further assumption. The fluids of the body that pass forward 

 carry nourishment for the head. When the worm is cut in two, 

 these substances are, in the posterior piece, stopped in their 

 forward movement by the cut surface, and accumulate at this 

 place. They act especially on the head germ, and, nourishing 

 it, bring about the development of the new head. Similarly, 

 fluids to nourish the tail are assumed to flow posteriorly; hence 

 in an anterior piece of a worm they will accumulate at its poste- 

 rior cut surface, and, acting on the tail germ, there bring about 

 the development of a new tail. 



In another species of fresh-water annelid Bonnet found (1745) 

 that when the worm was cut in two a new tail developed at the 

 anterior end of the posterior piece, and not a head. He sup- 

 posed that in this worm only tail germs are present throughout 



