BOOK IV. 



THE TISSUES AND MECHANISMS OF REPRODUCTION, 



CHAPTER I. 



ORGANS OF REPRODUCTION. 



775. MANY of the individual constituent parts of the body are capa- 

 ble 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 

 epithelial scales which fall away from the surface of the body are suc- 

 ceeded 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 reappear almost without limit; new secreting cells 

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

 tion of some cases, such as cartilage, and these doubtful 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 any- 

 thing 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 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 im- 

 perfect. 



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 spermatozoon, developed into an adult indi- 

 vidual. 



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

 connected with the series of changes through which the ovum becomes the 

 adult being ; or into the obscure biological inquiry as to how the simple, 

 all-but-structureless ovum contains within itself, in potentiality, all its future 

 developments, and as to what is the essential nature of the male action. 

 These problems and questions are fully discussed elsewhere ; they do not 



53 833 



