REGENERATION. 451 



and metabolism on contraction of bloodless muscles. Increased respiration and 

 apnea are not followed by increased oxidation. 



REGENERATION. 



The power of regenerating parts that have been lost varies widely in different 

 organs and tissues. It is much more marked in the low r er animals than in warm- 

 blooded animals. Division of the fresh-water polyp (hydra) is followed by the 

 development of two new individuals. An entire being may even develop from 

 every excised portion of the trunk of the body; only exceedingly small pieces give 

 rise to incomplete reproduction. No animal regenerates portions of the arm. 

 Also the planaria exhibit similar powers of regeneration. From every portion of 

 the umbrella of certain medusae (thaumantiades) , if it contain only a portion of 

 the margin, a new medusa may develop. From the surface of a piece of the trunk 

 of a turbellaria directed downward a pedal extremity develops, from the upper 

 surface a cephalic extremity, and if attached horizontally heads develop at both 

 extremities. Artificial division is possible also in rhizopods and infusoria. 

 Divided infusoria regenerate only if the divided portion contains a part of the 

 nucleus. Transversely divided earthworms (lumbriculus variegatus) regenerate 

 entirely to whole individuals. The decapitated head has been observed to re- 

 generate five times. In starworms the excised snout, together with the 

 pharyngeal ring of the central nervous system, regenerates. Spiders and crabs 

 regenerate feelers, legs and claws; snails, parts of the head, including feelers and 

 eyes, providing the central nervous system is uninjured. Some fish are capable 

 of replacing repeatedly destroyed fins, principally the caudal fin. Salamanders 

 and lizards exhibit regeneration of the entirely lost tail, with bones, muscles and 

 even the posterior extremity of the spinal cord. In young frogs amputated 

 legs regenerate, but only when the bones also are divided, and not after ex- 

 articulation. In tritons the lower jaw regenerates. In order, however, that this 

 regeneration shall take place a stump at least must be left. Total extirpation 

 of the parts mentioned destroys the power of regeneration. 



Loeb designates as heteromorphosis the phenomenon that occasionally after 

 injuries supernumerary parts appear that otherwise do not belong in such situa- 

 tions. Thus, for example, in young lizards lateral notching of the tail may cause 

 the growth of a second tail from the wound; likewise supernumerary extremities 

 develop in tailed amphibia after amputation. Planaria injured on the head exhibit 

 the growth of a second head. 



In amphibia and reptiles the regeneration of organs and tissues follows, on 

 the whole, the type of embryonal development, and the histological processes in 

 the growing caudal extremity and in regenerating portions of the body of earth- 

 worms take place in the same manner. In amphibia and reptiles only tissue of 

 the same kind develops from injured tissue. The spinal cord regenerates from 

 the epithelial cells of the central canal. In the process of tissue-formation the 

 leukocytes assume only the function of nutrition and conveyance of material. It 

 is a remarkable fact that tadpoles develop after destruction of the brain and the 

 medulla and functional exclusion of the spinal cord. 



The power of regeneration is much more restricted in warm-blooded animals 

 and in man. In these also it is confined principally to early life. True regenera- 

 tion is exhibited by 



1. The blood; first the plasma, then the white and finally also the red blood- 

 corpuscles. 



2. The epidermal structures and the epithelium of the mucous membrane re- 

 generate by cell-division in the deepest layers after previous nuclear division. 

 After direct loss they regenerate so long as the normal matrix upon which they 

 grow and the deepest layer of cell-protoplasm capable of development is not also 

 destroyed. In the latter event, regeneration ceases and restoration must take 

 place from the margins of the deficiency. In the process of regeneration, there- 

 fore, growth takes place always either from the deep layers, or, after their de- 

 struction, from the margins. There develop under such circumstances proto- 

 plasmic wandering cells that in part become detached and help to close the 

 deficiency, and in part the deepest layer of cells develops into large, multinu- 

 cleated protoplasmic cells, which multiply by division into polygonal flat nu- 

 cleated cells. 



The nail grows from the posterior fold forward, on the fingers in the course 

 of from four to five months, on the great toe in about twelve months (and more 



