CHAPTER XIX 

 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 peripheral nerve-fibres which 

 have been destroyed by accident or operation are readily regenerated, 

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

 generation. 



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. Artificial mouths surrounded by 

 tentacles can be formed in Cerianthus, an animal belonging to the 

 same group as the sea-anemones, merely by making a cut in the 

 body-wall and preventing it from closing. In an Ascidian, too (the 

 Cynone intestinalis) , artificial openings in the branchial sac, sur- 



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