GROWTH AND SENESCENCE 39 



with cessation of growth. The results of Carrel then lead to the 

 conclusion that body cells are not only potentially capable of in- 

 definite growth, but they are also potentially immortal, and that 

 cessation of growth, and death, is due to the finite nature of the 

 universe in which the organism or tissue finds itself. 



Having obtained some insight into the mechanism of simple 

 growth in bulk, we may mention the mechanisms of differentia- 

 tion. We have divided the life cycle into seven stages (there 

 may be many more), and each stage is characterized by some 

 special feature. The adolescent stage, for example, is charac- 

 terized by an acceleration of growth in stature, weight, etc. By 

 what mechanism are these changes brought about? The theory 

 of these changes is even less clearly understood than the theory 

 of growth in bulk. We are inclined to the belief that these 

 changes are brought about by substances of the nature of hor- 

 mones. The tissues from the following glands may be men- 

 tioned as possibly producing differentiating hormones: thyroid 

 (the thyroids, which weigh from one to two ounces, are situated 

 on each side of the windpipe) ; hypophysis, or pituitary glands 

 (situated at the base of the skull) j pineal gland (situated at the 

 base of the brain behind and above the pituitary) ; and the sex 

 glands. A detailed consideration of these, and other hormone- 

 producing glands and their activities will be found in Dr. 

 Green's lecture of this series. 



The three mechanisms of growth explained above, namely, 

 the inherent force residing in cells to grow indefinitely, the 

 growth-retarding influences resulting from the finite universe 

 in which the cells or organisms find themselves, and the hor- 

 monic mechanisms, are the internal factors of growth. There 

 are many external factors regulating growth, such as climate 

 and food supply. We shall assume in this discussion that the 

 external factors are optimal. The effects of food supply on 

 growth are discussed by Dr. Hogan in another lecture of this 

 series. 



In the above discussion, we have attempted to present the 



