GROWTH AND STRUCTURE OF HYPOPHYSIS IN RAT 343 



variation. At ihc otlier extreme is St 47.5 (at 67 days) in which 

 no mitoses were found in 3 entire sections. In this case the body 

 weight was unusually high (196 grams), however, and growth 

 had probably become very slow as the adult weight was ap- 

 proached. It is probable that occasional mitoses occur so long 

 as growth continues. 



b. Mitoses in young rats held at maintenance, and in adidts with 

 acute and chronic inanition 



In the young rats held at maintenance from the age of three 

 weeks to ten weeks or more, mitosis has nearly ceased (tables 

 5 and 6). No mitoses were observed in the partes intermedia 

 and nervosa. In the pars anterior, however, mitoses still occur 

 occasionally, the total number found in 24 sections being 6, or 

 an average of one-fourth per section. It is of interest to note 

 that occasional mitosis in the pars anterior persisted even in a 

 rat held nearly at maintenance from three weeks to twenty 

 weeks of age (St 38.8), and which was nearly dead from inanition 

 when killed. In spite of this persistent mitosis, however, the 

 pars anterior apparently suffers the greatest relative loss in 

 volume, as previously shown. 



No mitoses were observed in any of the older rats subjected 

 to acute and chronic inanition. No systematic search was made 

 for them over entire sections, however, so they are not included 

 in the tables. 



c. Mitoses in various stages of ref ceding after maintenance 



In the young rats refed after a period of maintenace (tables 

 5 C and 6), mitoses were never found in the pars nervosa. In 

 the pars intermedia, in rats refed one to four weeks, they occur 

 occasionally, about as often as in younger normal I'ats of corre- 

 sponding weight. In the pars anterior, mitosis reappears 

 promptly. In rats refed only one-half week, in 4 sections ex- 

 amined a total of 7 mitoses was observed (table 5 C 1), or an 

 average of one and three-fourths per section. The rate increases 

 to an average of about 7 mitoses per section in rats refed one 

 week and two weeks, and about 3 in those refed four weeks. 



