338 C. M. JACKSON 



dimension is greatest in less than half of the cases. It will be 

 noted, however, that the parathyroid is frequently nearly spheri- 

 cal in form, the three axes being approximately equal (table 1: 

 figs. 1 and 3 to 6). 



The normal growth in the dimensions of the parathyroid is 

 evident from the table. In the pair of newborn parathyroids, 

 the average diameter is 0.23 mm.; at 2 weeks it is 0.28 mm.; 

 at 3 weeks 0.35 to 0.44 mm.; and at S weeks, 0.56 mm. In 5 

 rats from 70 to 74 days of age, the average is 0.54 mm. (range 

 0.48 to 0.67 mm.). The dimensions increase very irregularly up 

 to 0.90 nnn. average in a rat of 322 g. gross body weight, 442 

 days of age. 



In spite of the comparatively small number of observations 

 and the admitted inadequacy of the average diameter as a meas- 

 ure of the size of the parathyroid, it is believed that these data 

 are useful for purposes of general comparison. The data for 

 the individual parathyroids were compared with the weights of 

 the corresponding thyroids (not shown in the table), but there is 

 apparently no close correlation between them. 



A point of special interest, which is not apparent from casual 

 inspection of the table, appears more clearly when the observa- 

 tions are plotted graphically according to the body weight and 

 average diameter of the parathyroid. The gland appears to be 

 relatively larger in the females. Further observations will of 

 course be necessary to establish this point with certainty; but 

 if true, this will place the parathyroid in a class with the hy- 

 pophysis and suprarenals, in which the female rats possess rel- 

 atively larger glands. 



The normal structure of the parathyroid gland in the albino rat 

 at 3 weeks of age is shown in figure 11. The epithelial cells are 

 grouped in irregular, closely-packed masses, occasionally showing 

 a cylindrical or cord-like arrangement. 



In size the parathyroid epithelial cells vary considerably, the 

 average being about 6x8 micra, or slightly larger. They are 

 thus considerably smaller than the thyroid epithelium of a cor- 

 responding gland. The cell boundaries are usually distinct. 



