296 ELEMENTS OF BIOLOGY 



but is very low in most of the Nemathelminthes. Annelida and 

 Echinodermata, or at least certain members of these phyla, are able 

 to regenerate a whole from a part, but regeneration in Arthropoda 

 is limited to the replacement of appendages by some members of 

 the group; for example, a new claw will develop on the leg of a 

 lobster when one has been broken ofif. In some of the lower mem- 

 bers of the phylum Chordata, the sea squirts, a separated portion 

 may regenerate the whole animal. Among the Amphibia, some 

 salamanders (tailed Amphibia) will regenerate a lost leg, but the 

 frogs and toads (tailless Amphibia) are unable to do so. In the 

 higher animals only certain tissues retain the power of regenera- 

 tion. In the human body, the epidermis, the blood, and a few other 

 tissues are capable of regeneration. 



In forms like planarians, nemertians, and similar simple forms 

 that will regenerate a whole animal from a small fragment, the 

 processes of regeneration are in some respects like those of the 

 development of an adult from an embryo, although the tissue at 

 the start of regeneration is adult, that is, old tissue. Thus to begin 

 with, regeneration in such forms involves a renewal of the char- 

 acters of young animals, a rejuvenescence. In general, animals 

 which possess the power of regenerating a whole animal from a 

 piece are also animals that reproduce asexually, by budding or frag- 

 mentation, but this is by no means invariably true. For example, 

 Phagocata gracilis, a small flatworm closely similar to Planaria re- 

 produces solely by the sexual process of fertilization of eggs by 

 sperms; but a very small portion of the adult animal will regenerate 

 into a normal whole. 



If a cross piece is taken from the body of hydra or from the body 

 of a planarian, the new hydranth or the new head develops at the 

 end which was most anterior in the whole animal. In other words, 

 the piece retains the polarization that it possessed while in the intact 

 animal, the same relation of dominance of the anterior over the 

 more posterior regions. The principle of dominance and subordina- 



