342 



C. M. JACKSON 



case. Clrowtli in the purs iiei'A'osa thereafter consists abuost 

 entirely in the accumulation of intercellular substances. 



In the pars intermedia mitosis continues, but at a progres- 

 sively decreasing rate. The decrease is even relatively greater 

 than is apparent, on account of the progressive increase in the 

 absolute size of the areas examined. The average of 9 mitoses 

 per section in the newborn decreases to a little less than 1 at 

 one week and three weeks, and to one-thirteenth at ten weeks. 



TABLE 6 



Summarij of number of miloses in sections of the hypophysis. Averages from table 

 5 {exceptional cases J 1.3 and St 11.46 being excluded) 



It apparently increases somewhat in the older cases (even exclud- 

 ing J 1.3, which also shows an abnormally large number in the 

 pars anterior), bringing the average up to one-seventh. This 

 apparent increase is probably due to chance variation in the 

 relatively small number of sections examined. 



In the pars anterior, the rate of mitosis likewise decreases 

 (tables 5 and 6), the average number being 62 at birth, 18 at 

 one week, 7 at three w^eeks, 2 at ten weeks and five-sevenths in 

 3 older rats. From the last group is excluded one very excep- 

 tional case (J 1.3, 94 days) in which the number of mitoses varied 

 from 8 to 12 per section. This represents an extreme individual 



