IQ4 BIOLOGICAL LECTURES. 



less animals of all classes in the endeavor to discover whether 

 the same power of regeneration existed in other forms. Bonnet 

 (1745) and Spallanzani (1768) were most successful with their 

 experiments made on fresh-water worms, earthworms, the limbs 

 of newts, and the heads of snails (Helix). The results of the 

 experiments on snails, made by Spallanzani, called forth the 

 most vehement criticism from his contemporaries ; for many 

 of them seemed to consider the discovery that a snail could 

 regenerate its head and all the contained organs was beyond 

 credence. No fewer than eleven writers who had themselves 

 repeated this experiment (in one case on 1400 snails) affirmed 

 that when the head was completely cut off no regeneration took 

 place. On the other hand, as many as ten experimenters con- 

 firmed Spallanzani's results. Bonnet championed Spallanzani's 

 cause, and the weight of his authority did much toward the 

 acceptation of the latter's results. 



Similar discussions have arisen from time to time in regard 

 to the power of regeneration of many other forms, and the 

 fact is of interest as showing that the conditions of the ex- 

 periment, as well as the condition of the animal, are of im- 

 portance in deciding whether or not regeneration will take 

 place. 



The majority of naturalists, at the time of which I have just 

 spoken, were content to wonder at the results ; a few, however, 

 took a philosophic interest in the phenomena, and attempted 

 to bring them into line with the current theories of genera- 

 tion. The speculations of Spallanzani and of Bonnet are of 

 great interest, and not dissimilar from certain modern attempts 

 to explain the process of regeneration by the preformation 

 theory. 



With the development of our modern technique a new interest 

 was awakened in the phenomenon of regeneration, particularly 

 in connection with the cellular and germ-layer theories. Nu- 

 merous attempts have been made to discover the origin of the 

 new cells in the restored part, and to determine whether the 

 newly formed organs arise from the same germ-layers that give 

 rise to similar organs of the embryo. It is instructive to ob- 

 serve how often the facts have been interpreted so as to bring 



