No. 643] SEROLOGICAL PHENOMENA 



129 



trophy is carried by the blood, although the possible influ- 

 ence of the nervous system must also be reckoned with 

 in higher animals. And if there is such a serum-borne 

 agent in the case of compensatory hypertrophy, may 

 there not also be one in that of the ordinary physiolog- 

 ical hypertrophy of the exercised muscle or stressed 

 bone? If so, we must keep our minds open to the pos- 

 sibility that it may also stimulate the germinal proto- 

 types of such proteins to additive functioning. For if 

 we had but a single side-chain in common between a pro- 

 tein of somatic tissue and a protein of the germ, any- 

 thing that could affect one might well be expected to 

 affect the other. 



Again, mechanical stimuli, if not too severe, and vari- 

 ous irritative substances in amounts sufficiently small not 

 to be destructive or poisonous to the tissues, may stimu- 

 late cells to overgrowth. Verj' small amounts of arsenic 

 or phosphorus, for example, may thus affect the kidneys 

 and liver, and minute doses of phosphorus may cause in- 

 creased growth of bone. In such cases also it is probable 

 that serological changes are involved. This is almost 

 certainly true in cases of vicarious overgrowth, where an 

 organ supposedly of related function takes over wholly 

 or in part the work of another tissue. An example of 

 this is seen in the enlargement of the pituitary gland 

 when the thyroid is atrophied or removed, or the com- 

 pensatory enlargement of the hemolymph glands and 

 bone-marrow follow^ing removal of the spleen. Still 

 further may be cited the phenomena of metaplasia, in 

 which, through modification of function and nutrition, 

 specialized tissues develop from cells w^hich normally pro- 

 duce tissue of another order. It sometimes happens, for 

 example, that the choroid coat of a severely injured eye, 

 after the lapse of considerable period of time, will de- 

 velop a layer of true bone. In fact, metaplastic foi-ma- 

 tion of bone is common in many tissues. Such facts 

 show that many, if not all, tissue-cells have the capacity 

 to form very different kinds of tissue in different en- 

 vironments, and suggest that they retain all the inheren- 



