236 Regeneration. 



nervous cells and striated muscle, which can only exist, from a 

 functional standpoint, in connection with nerves and which are of 

 a decidedly complex structure, and in the same way glandular cells, 

 regenerate only under special circumstances or not at all. 



[The conditions, then, which underlie cellular regeneration may 

 be summarized somewhat as follows: Only cells which from the 

 simplicity of their structure and function or from the fact that they 

 have not advanced far from their embryonic state are likely to mul- 

 tiply with any comparative ease. Any cell must be provided with 

 sufficient nutrition to maintain its cellular activity and life if it be 

 expected to divide, and extra nutrition as met in hypersemia favors 

 multiplication. Cells capable of division cannot, however, prolif- 

 erate if there be important mechanical or physiological opposition 

 exerted by the other cells and structures of the body, and release 

 of such mutual tension (mechanical, nervous, chemical) must be 

 afforded before proliferation is possible. But in addition a definite 

 stimulus to division should exist. This stimulus may be of chem- 

 ical nature, perhaps afforded by special chemical substances coming 

 to the part from previous cellular disintegration (here the brilliant 

 results of Jacques Loeb in producing multiplication and develop- 

 ment of unfertilized ova of sea-urchins by immersion in special 

 chemical solutions are strongly corroborative), electrical (here, too, 

 the work of Loeb and his associates is illuminating), thermic or 

 mechanical.] 



In the adult organism of animals and birds the somatic cells 

 reproduce in their process of division and multiplication only cells 

 of their own type (omnis celhila e ccUula cjusdem generis) ; in other 

 words, a newly formed tissue is invariably the product of tissue of 

 the same kind. It was formerly believed the various types of cells 

 could in their multiplication produce opposite types and that in the 

 same way as the various organs arise by their differentiation from 

 the three embryonic layers similar transformations of tissue could 

 take place in the restoration of post embryonic defects. A certain 

 capacity for transformation (metaplasia) is possible in related 

 types of cells and their matrix products. Thus the different 

 varieties of epithelial cells may be changed by external influences 

 (pressure) ; and within the group of connective tissues transforma- 

 tions occur, as the formation of bone and cartilage from fibrous 

 connective tissue. But connective tissue is never changed into 

 epithelium, or the latter into connective tissue, the four distinct 

 groups of primary tissues (epithelium, connective tissue, nervous 

 tissue and muscle) never interchanging. 



