692 TEXTBOOK OF ZOOLOGY 



Muscle Tissue. — Smooth muscle has slight regenerative capacity. 

 Healing is effected largely through the formation of fibrous tissue. 

 In the case of striated muscle, sareolemma cells sometimes divide 

 after injury and a limited amount of regeneration takes place. 



Nervous Tissue.— Nerve fibers do not reunite after they are 

 divided. The distal portion of the severed fiber degenerates com- 

 pletely. The neurilemma cells undergo proliferation and unite with 

 the neurilemma of the proximal portion. The axone may then grow 

 into the united sheath. The process takes months. Nerve cells of 

 adult mammals do not proliferate. 



Basis for Regeneration 



Certain facts must be reviewed to furnish a basis for an explana- 

 tion of regenerative capacity. Differentiation of body cells during 

 embryonic development gives rise to an integrated organism. Genetic 

 experiments indicate that every cell in the body, with sporadic ex- 

 ceptions, contains the same genes as every other cell. Differentiation 

 does not consist of segregation of genes for the various organs. 

 Different expressions of the same genes at different times in devel- 

 opment or at different places in the organism or the expression of the 

 effects of particular genes only at a particular time and place in the 

 organism must be responsible for differentiation of organs and tissues. 



The zygote is capable of giving rise to a whole organism. When 

 the zygote of some organisms divides, each of the two cells produced 

 is capable of producing a whole organism, if separated from the 

 other, regardless of the complexity of the adult organism ; e.g., in the 

 sea urchin and in man. Such a separation is indicated in the case of 

 identical twins. In other animals, due to differentiation of the cyto- 

 plasm, the first cleavage reduces the potency of the resulting cells so 

 that they will not produce a whole organism if they are separated. 

 Obviously, if the first two cells remain in normal position, each pro- 

 duces but a half organism in any case, demonstrating the importance 

 of the relationship between cells in limiting the potency of each. The 

 first two cells formed by division of the zygote are thus physiologi- 

 cally different though genetically identical so long as they remain in 

 their normal relationship to each other. As development and dif- 

 ferentiation proceed, potencies of the cells tend to be limited more 

 and more, the final limitation of potencies in the adult being slight 

 in simple organisms, great in complex organisms. 



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