22 GEORGE T. HARGITT 



it. He found, for example, that the regenerating organs in 

 annelids came partly from the old organs and partly from new 

 sources; new muscles came, not from old muscle or even from 

 mesoderm, but from the ectoderm, the pharynx regenerated 

 from entoderm instead of ectoderm as in the original development. 

 Other evidence of the same sort was directly contradictory to 

 the view that regeneration is due to latent germ cells. Morgan 

 ('01, '07) believes regeneration is a growth process. Schultz ('02) 

 thinks regeneration is a primary property of life, limited more or 

 less in consequence of specialization of tissues, but always poten- 

 tially present. His conclusion is in accord with that of Morgan, 

 and implies development, budding, and regeneration to be 

 exhibitions of the capacity for growth inherent in all protoplasm. 

 Montgomery ('06) and Hegner ('14) reject this view and accept 

 the germ-cell explanation, the latter stating that regeneration in 

 Coelenterates is always due to widely distributed germ cells. 

 C. W. Hargitt ('11) points out serious objections to this expla- 

 nation in hydroids, and Hegner admits the impossibility of 

 accounting for regeneration of sex organs on this view. But sex 

 organs are readily regenerated in hydroids. Child ('15) has 

 observed that specialized cells of Pennariia may undergo a de- 

 differentiation and take part in budding, along with the inter- 

 stitial cells; the same thing occurs during regeneration. Morgan 

 has also found abundant evidence of the formation of masses of 

 indifferent cells by regressive changes, and the production of new 

 structures from such masses in regeneration. Morrill ('18), 

 working upon the regeneration of appendages in salamanders, 

 observed the formation of masses of cells by simplification of old 

 specialized cells, and the differentiation of muscle and cartilage 

 from these cells. 



In Hydra and hydroids regeneration may take place at practi- 

 cally any point where a cut is made, and almost as often as new 

 growths are excised. Very minute pieces may also regenerate 

 complete animals, normal in all respects, including reproductive 

 organs. The minimal size is always a group of cells, and yet, 

 according to the theory of regeneration from germ cells, there is 

 no reason why a single cell might not produce a new organism. 



