352 G. HAROLD DREW 



in vertebrates, he has observed epithelial cells migrate into the 

 underlying tissues, and take on the appearance of fibroblasts. 



Much of the work that has been done on regeneration is of 

 interest in this respect. Braun ('03) has pointed out that in re- 

 generation in tadpoles, epithelium may give rise to nerve tissue. 

 There is the work of Barfurth('91-'00) and Fraisse( '85) on regenera- 

 tion of the tail in several Urodela, and in the tadpole of the frog, 

 and of Towle on the limbs of Plethedon. These observers find that 

 each tissue only reproduces tissue similar to itself, but their experi- 

 ments were necessarily confined to types in which regeneration 

 occurs, and dealt only with the phenomena consequent on amputa- 

 tions. Much more work has been done on the regeneration of 

 single tissues or organs, and in these cases it has been found that 

 like always reproduces like, and that metaplasia does not occur. 



To turn now to invertebrates, we have the observations of Miss 

 Reed, recorded by Morgan, ('04) on the regeneration of the claw 

 of the crayfish and hermit crab, in which it is conclusively shown 

 that the whole claw with its muscles, etc., is regenerated from the 

 ectoderm, i.e. metaplasia of ectodermal into mesodermal cells 

 occurs. Again there are the experiments of Koeber ('00) on the 

 regeneration of the pharynx of AUolobophora, in which he shows 

 that though the epithelium lining the pharynx is developed from 

 the ectoderm, yet in the course of regeneration it grows from the 

 endoderm continuous with that lining the alimentary canal. 



Flexner ('98) has pointed out that in regenerating Planarians 

 the surface epithelium may give rise to distinct nerve elements, 

 and in this case, though both the surface epithelium and the nerve 

 tissue are originally derived from the same germ layer, yet we 

 have metaplasia occurring between tissues which are histologic- 

 ally totally different, and indeed the process may in some ways be 

 considered analogous to the regeneration of the lens from the iris 

 in the eve of the larval newt and salamander described by Wolff 

 ('94). 



It thus appears that, whatever may be the case in vertebrates, 

 in lower types of animals there is much evidence to show that me- 

 taplasia may occur between cells derived from different germ 

 layers, and between cells possessing widely different histological 



