Studies of Tissue Growth 46^ 



limbs, was not only less than the length of the molting period for 

 the regenerating specimens, but also in a large proportion of cases 

 was even shorter than the molting period for the normal lobster, " 

 Emmel finally concludes, that the process of regeneration by retard- 

 ing both the frequency of molting and the increase in size retards 

 the growth of lobsters. 



After an examination of the relation between the increase in 

 size in normal and in regenerating salamanders Morgan states 

 C06): 



"That a newly regenerating part has the power to take from 

 the blood the materials that it needs for growth, even when the 

 amount present in the blood has fallen so low that the rest of the 

 tissues cannot maintain themselves, but break down to supply 

 the blood with a certain amount of nutriment. If this idea 

 expresses approximately the relation that exists, it follows that 

 while the new part requires a certain amount of food in order to 

 continue growing, it can take advantage of a condition that the 

 older or differentiated tissues cannot make use of; in fact, when 

 the latter slowly lose ground .... Since in regeneration 

 the new part is formed directly out of the old tissues we may 

 assume that this property (excessive capacity of assimilation) of 

 young parts is something connected with their lack of differentia- 

 tion, which is lost when differentiation takes place, and is regained 

 again when the differentiation is lost." 



Morgan's ideas are most suggestive when considered from the 

 standpoint of the conditions found in malignant grow^ths. In 

 Ewing's ('08) survey of the latter subject he calls attention to the 

 fact that the energies of cells are normally divided between pro- 

 liferation and specialized function, between work and growth, 

 both being limited by blood supply. Examples of cells set apart 

 for growth are the germ center cells of lymph nodes, the cells at 

 the bases of intestinal villi and the basal cells of the epidermis. 

 Ewing states that "it is just from these cells, subject to marked 

 variations of the demands for growth, that tumors arise. It is 

 clear that deficient demands for function on the part of derived 

 cells would leave their energies unconsumed and further available 

 for grow^th. These conditions surround the inception of cancer 

 in the atrophying breast." 



