CHAPTER XIX 

 REPRODUCTION 



Regeneration of Tissues. In lower forms of animals and in all 

 or most plants, the power of regeneration is much greater than in 

 the higher animals and in man. A newt can reproduce an am- 

 putated 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 formedjn Cerianthus, an animal 

 belonging to the same group as the sea-anemonss, merely by 

 making a cut in the body-wall and preventing it from closing. In 

 an Ascidian, too (Cynone intestinalis), artificial openings in the 

 bianchial sac, surrounded by numerous pigmented points similar 

 to the eye-spots around the natural mouth and anus, have been 

 produced (Loeb). A classical example of regeneration in inverte- 

 brates is that of the rays of starfishes after amputation. According 

 to some observers, this massive regeneration, as well as other in- 

 stances of the regeneration of complicated organs in invertebrates, 

 depends, in some degree at any rate, upon the nervous system. 



Even in the higher animals regeneration of tissues is a common 

 phenomenon. 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 



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