i 9 2 AGE, GROWTH, AND DEATH 



of teeth, in the spinal cord, and in many other cases. 

 In all of these instances the focus of cell production 

 might be described aptly as a growth-zone. In many 

 other instances the growth centres are very small. For 

 example, at the root of every hair there is such a 

 centre. The hair grows exclusively at its base ; the 

 steady addition of new cells forces the older ones up, 

 lengthening the hair. Every little gland in the stomach 

 and intestines has its own growth centre. Each of the 

 glands in question is a minute tube from one to two 

 millimetres in length, with its orifice (Fig 65), at the 

 inner surface of the digestive tract; the opposite end, 

 or base, ends blindly. We have now before us a picture 

 of a gland from the large intestine of a cat. It is seen 

 in vertical section. The irregularly shaped dark spots, 

 mi, represent dividing nuclei, and they are all located 

 near the bottom of the gland. 1 As the new cells are 

 produced the older ones are shoved up towards the 

 orifice and undergo differentiation. After getting to 



or appositional growth. This investigation was one which I suggested to Dr. 

 Schaper, while he was a member of my staff at the Harvard Medical School, and I 

 pointed out to him the difference involved between the two types of growth and 

 the bearing of it on the general notions which I had already formed and in part 

 published at that time. I also called his attention to a number of the specific 

 illustrations which he and Cohen studied. I regret that Dr. Schaper has failed in 

 this paper to mention the actual source of the general conceptions which he has 

 put forward as original with himself. The research by Schaper and Cohen is an 

 excellent piece of work and contains much that is new. 



1 The manner in which intestinal and gastric glands grow was discovered by 

 Bizzozero, "Sulleghiandoli tubulari del tubo gastro-enterico, etc., Nota prima," 

 Atti Accad. Sci. Torino, xxiv., 110-137, Tav. III. (1888). lie has also written 

 several subsequent articles, see especially A rchiv. fur mikrosk. Anat., xlii., 

 82-152, Taf.VII.-IX. His observations have been abundantly confirmed by later 

 investigators. 



