194 Biology in America 



problem, with unfortunately widely divergent results. The 

 ectoderm has been eut around the developing primary vesicle, 

 and the tlap folded back so as to expose the latter; vidiich has 

 then been excised, the tiap replaced and the wound allowed 

 to heal. In other experiments the primary vesicle has been 

 supposedly destroyed by pricking it with a hot needle; and 

 in still others the vesicle has been transplanted to a strange 

 area of the same, or a different species, such as the abdominal 

 wall. In the latter experiments lenses have been formed from 

 parts of the ectoderm which never give rise to them in nature, 

 and similar results have been obtained by destroying a lens 

 already formed, thereby causing its regeneration from the 

 iris. 



In the former experiments the results have been incon- 

 sistent, a lens sometimes regenerating and sometimes failing 

 to do so, after the removal of the primary vesicle. Werber 

 has suggested that this apparent inconsistency is due to the 

 incomplete destruction of the vesicle in some cases in which 

 it had supposedly been entirely removed, and the consequent 

 formation of a " lens stimulus ' ' by small pieces of the vesicle 

 which remained. Of interest in this connection are the experi- 

 ments of Stoekard, Werber and others in the production of 

 Cyclopean and other monsters, which will be considered later. 

 In some of these experiments a single median eye has been 

 produced in place of two lateral ones, with the resultant 

 formation of a single lens associated Avith the single eye, and 

 the absence of any lateral lenses. In other cases lenses have 

 developed at almost any place on the monster, apparently 

 unassociated with any optic material. Werber has suggested' 

 however that these so-called "independent lenses" owe their 

 origin to the stimulus of microscopic bits of optic vesicles 

 scattered over the body of the monster, through a process of 

 blastolysis or tissue destruction induced by chemical or 

 osmotic action, and in some cases he is able to demonstrate 

 what he considers bits of such material in close proximity 

 to these lenses. 



Whatever the truth of the matter may be, the evidence is 

 I think conclusive that lenses, and presumably other organs 

 also, are not iji any sense preformed, but result from the 

 interaction between the parts of the organism itself and their 

 environment. 



Nearly related to experiments on regeneration are those on 

 grafting. The custom of grafting in plants has been prac- 

 tised by horticulturists for a long time. Trembley with his 

 celebrated woik on Hydra was a pioneer in this field among 

 animals. JMoi'e recently this work has been continued by 

 King, Rand, Peebles and others in this country. The an- 

 terior end of one Hydra may be grafted onto the posterior 



