202 BIOLOGICAL LECTURES. 



root. In the one case, where the upper end corresponds to the 

 proximal end of the original hydroid, the new part (a stem) 

 replaces the lost root end, and this change Loeb calls heteromor- 

 phosis. In the other case, where the upper end corresponds 

 to the distal end of the original hydroid, the new part (a stem) 

 replaces the lost part of the stem. This is what is usually 

 meant by regeneration. It would be most instructive to find 

 out what pieces of this hydroid would do were they fixed to a 

 revolving wheel so arranged that the direction in which gravity 

 acts on the piece would change at every instant. 



It is of the greatest importance to note that in all these cases 

 described by Loeb, in which regeneration is determined by 

 external circumstances, the same influences affect the growth 

 of the uninjured hydroids in the same way. Only those forms 

 whose normal growth is influenced by light, or by gravity, or 

 by contact, could be induced to respond to the same influences 

 during the time of regeneration. 



In review of the few facts that we possess in regard to the 

 effect of external factors, we must bear in mind that in some 

 cases the external agent, gravity for instance, does not seem in 

 itself necessary for the growth of the animal, but it acts in such 

 a way that it determines what sort of differentiation may take 

 place. Other factors, temperature for example, determine only 

 whether or not growth can take place at all, and if so, the 

 rapidity of the growth. The salts contained in the water act 

 in the same way, in so far as they are necessary for the metab- 

 olism of the cells. In so far as the salts affect the osmotic 

 condition of the cells they seem to determine — • as Loeb has 

 shown for these hydroids — the amount of growth, but not its 

 kind. It may be that other factors, light for instance, may 

 affect not only the amount, but also the kind of growth. 



In this same connection I should like to describe an ingenious 

 experiment made by Herbst. He found that when the eyes 

 of a prawn were cut off, sometimes an eye and sometimes an 

 antenna rescenerated at the cut surface. He then tried the 

 experiment of placing an equal number of new individuals — 

 after cutting off the eyes — in the light and in the dark, to see 

 if the light would act as a stimulus on the forms in the light, and 



