EXPERIMENTAL MORPHOGENESIS 105 



It may be simple mechanical contact, or it may be some 

 chemical influence that really constitutes the " stimulus " in 

 this case ; certainly, there exists a close and very specific 

 relation of the localisation of one part of the embryo to 

 another. Things are much the same in another case, 

 which, after having been hypothetically stated by Herbst 

 on the basis of pathological data, was proved experimentally 

 by Spemann. The lens of the eye of certain Amphibia is 

 formed of their skin in response to a formative stimulus 

 proceeding from the so-called primary optic vesicle. If this 

 vesicle fails to touch the skin, no lens appears ; and, on the 

 other hand, the lens may appear in quite abnormal parts of 

 the skin if they come into contact with the optic vesicle 

 after transplantation. 



But formative dependence of parts may also be of 

 different types. 



We owe to Herbst the important discovery that the 

 eyes of crayfishes, after being cut off, will be regenerated in 

 the proper way, if the optic ganglion is present, but that 

 an antenna will arise in their place if this ganglion has 

 also been removed. There must in this case be some 

 unknown influence of the formative kind on which 

 depends, if not regeneration itself, at least its special 

 character. 



In other cases there seems to be an influence of the 

 central nervous system on the regenerative power in general. 

 Amphibia, for instance, are said to regenerate neither their 

 legs (Wolff), nor their tail (Godlewski), if the nervous com- 

 munications have been disturbed. But in other animals 

 there is no such influence ; and in yet others, as for instance, 

 in Planarians, it must seem doubtful at present whether the 



