CHAPTER XIV. 

 REPRODUCTION. 



Regeneration of Tissues. Since cells are constantly dying within 

 the body, they must be constantly reproduced. In some tissues the 

 process by which this is accomplished is more evident, and therefore 

 better known, than in others. The most highly-organized tissues are 

 with difficulty repaired, or not at all. The epidermis is always 

 wearing away at its surface, and is being constantly replaced by the 

 multiplication of the cells of the stratum Malpighii. In the corneous 

 layer we have only dead cells ; in the Malpighian layer we have every 

 histological gradation from squames to columns, and every physio- 

 logical gradation from cells which are about to die to cells that have 

 just been born. The corpuscles of the blood undoubtedly arise at 

 first, and are recruited throughout life, by the proliferation of mother- 

 cells. The gravid uterus grows by the formation of new fibres from 

 the old, and by the enlargement of both old and new. A severed 

 muscle is generally united only by connective or scar tissue, but 

 under favourable conditions a complete muscular * splice ' may be 

 formed. A broken bone is regenerated by the proliferation of cells 

 of the periosteum, which become bone-corpuscles. We do not know 

 whether there is any new formation of nerve-cells in the adult 

 organism (but see p. 749), but peripheral nerve-fibres which have been 

 destroyed by accident or operation are readily regenerated by the 

 growth of new processes from the cells that originally produced them ; 

 and the end-organs of efferent nerves may share in this regeneration. 



In lower forms of animals, and in all or most vegetables, the power 

 of regeneration is much greater than in man. The starfish can not 

 only repair the loss of an arm, but from a severed arm a complete 

 animal can be developed. A newt can reproduce an amputated toe, 

 and every tissue skin, muscle, nerves, bone will be in its place. 

 After extraction of the crystalline lens in triton larvae, a new lens is 

 formed from the iris epithelium. 



Thus, in a sense, reproduction is constantly going on within the 

 bodies even of the higher animals. But since the whole organism 

 eventually dies, as well as its constituent cells, a reproduction of the 

 whole, a regeneration en masse, is required. 



A cell of the stratum Malpighii can only, so far as we know, 

 reproduce a similar cell, and this is characteristic of cells that have 



