REPRODUCTION. 



923. 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 during 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 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 exceptions, all those fundamental 

 tissues of the body which do not form part of highly differ- 

 entiated organs are, within limits fixed more by bulk than by 

 anything else, capable of regeneration. To that regeneration by 

 substitution of molecules, 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 individual 

 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 representative or sperma- 

 tozoon, developed into an adult individual. 



We do not purpose to enter here into any of the morphological 

 problems connected with the series of changes through which the 



