THE TISSUES AND MECHANISMS OF REPRODUCTION. 



MANY of the individual constituent parts of the body are capable 

 of reproduction, i.e. they can give rise to parts like themselves; or 

 they are capable of regeneration, i.e. their places can be taken by 

 new parts more or less closely resembling themselves. The ele- 

 mentary tissues undergo during life a very large amount of re- 

 generation. Thus the old epithelium scales which fall away from 

 the surface of the body are succeeded by new scales from the 

 underlying layers of the epidermis ; old blood-corpuscles give place 

 to new ones ; worn-out muscles, or those which have failed from 

 disease, are renewed by the accession of fresh fibres; divided nerves 

 grow again ; broken bones are united ; connective tissue seems to 

 disappear and appear almost without limit ; new secreting cells 

 take the place of the old ones which are cast off; in fact, with the 

 exception of some cases, such as cartilage, and these doubtful ex- 

 ceptions, all those fundamental tissues of the body, which do not 

 form part of highly differentiated organs, are, within limits fixed 

 more by bulk than by anything else, capable of regeneration. 

 That regeneration by substitution of molecules, which is the 

 basis of all life, is accompanied by a regeneration by substitution 

 of mass. 



In the higher animals regeneration of whole organs and mem- 

 bers, even of those whose continued functional activity is not 

 essential to the well-being of the body, is never witnessed, though 

 it may be seen in the lower animals ; the digits of a newt may be 

 restored by growth, but not those of a man. And the repair 

 which follows even partial destruction of highly differentiated 

 organs, such as the retina, is in the higher animals very imperfect. 



