g6 THE GERM-PLASM 



Many tissues, even in the highest animals, when they have 

 suffered an abnormal loss of substance, are renewed in precisely 

 the same way as in the cases of physiological regeneration 

 already mentioned. Thus in mammals, for instance, portions of 

 muscular tissue, of epithelium covering an organ or lining the 

 duct of a gland, and of bone, can be replaced by cellular elements 

 of a similar kind ; and recent researches in pathological anatomy 

 render it almost certain that all these regenerative processes orig- 

 inate in the cells of the tissue which is to be replaced. Hence 

 these tissue-cells retain the power of multiplying by division, but 

 they only begin to exercise this power in response to certain 

 external stimuli, more particularly to that which is produced by 

 a loss of substance in their immediate vicinity. Thus epithelial 

 cells multiply around a defect in the epithelium ; and in an 

 injured muscle, the nuclei multiply and cause the surrounding 

 protoplasm to be transformed into cells, which become spindle- 

 shaped, and give rise to muscle-fibres. In both these cases we 

 must merely attribute to the idioplasm the capacity for multipli- 

 cation : the cells in question only begin to divide when influ- 

 enced by a stimulus due to the loss of substance, or, as it would 

 be expressed in the language of modern pathology,* 'by the 

 removal of the resistances to growth.' Thus in these very simple 

 cases of the abnormal loss of parts, the rest of the tissue gives 

 rise to a stock of determinants from which replacement of the 

 part can occur. 



In more complicated tissues, the process of regeneration is 

 less simple. Thus Fraisse has shown that in the Amphibia 

 the entire epidennis, toi^et/ier wiiJi the sliine-glands and the 

 integumentary sense-organs, is regenerated by the epidermic 

 cells in the vicinity of the defect. In this case also, the new 

 material is furnished by the deeper uncornified layers of the 

 epidermis. But the newly-formed cells do not all develop into 

 the same kind of tissue. The main mass of them gives rise to 

 the stratified epidermis, while others ' unite to form pearl-shaped 

 masses in the deeper part of the epidermis, the cells becoming 

 grouped around an imaginary centre.' ' Connective tissue-cells 

 then migrate from the cutis, and these masses, each consisting of 

 from ten to twenty cells, thus become marked off from the epi- 

 dermis.' 'At the same time pigment-cells wander into the 



* Cf. E. Ziegler, ' Lehrbuch der pathologischen Anatoniie,' Jena, 1890. 



