REPRODUCTION. 



§ 686. Many of the individual constituent parts of an 

 animal 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 elementary tissues undergo dur- 

 ing life a very large amount of regeneration. 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 re- 

 newed by the accession of fresh fibres ; divided nerves grow 

 again ; broken bones are united ; connective tissue seems to dis- 

 appear 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 doubt- 

 ful exceptions, 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. To that regeneration by substitution of mole- 

 cules, which is the basis of all life, is added a regeneration by 

 substitution of mass. 



In the higher animals regeneration of whole organs and 

 members, 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. 



In the higher animals the reproduction of the whole in- 

 dividual can be effected in no other way than by the process of 

 sexual generation, through which the female representative 

 element or ovum is, under the influence of the male representa- 

 tive or spermatozoon, developed into an adult individual. 



We do not purpose to enter here into any of the morpholog- 



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