POSTNATAL GROWTH IN THE ALBINO RAT 39 



13. Gonads 



a. Testes. The relative (percentage) weight of the testes, cal- 

 culated from Hatai's (13) formulas, is represented by the curve 

 in figure 7 a. This gives a relative weight of 0.40 per cent of 

 the body weight at 10 grams and about the same at 20 grams. 

 Unlike that of other organs, the relative weight of the testes con- 

 tinues to increase for a long time (up to age of puberty), being 

 0.80 per cent at 50 grams and reaching a maximum of 1.30 per 

 cent from 90 to 100 grams. Thereafter it decreases to 1.28 per 

 cent at 120 grams, 1.16 per cent at 170 grams, 1,09 per cent at 

 200 grams, 0.90 per cent at 300 grams, and 0.76 per cent at 400 

 grams. 



Unfortunately, in my own data the epididymis is included with 

 the testes and the results are therefore not strictly comparable 

 with those of the formula above, which do not include the epi- 

 didymis. A few special ijieasurements indicate that the epididy- 

 mis fomis one-fifth or one-sixth of the weight of the testes proper 

 in the younger rats, which however increases (with irregular vari- 

 ations) to one-third after the age of puberty, at about ten weeks. 

 In extreme cases, the weight of the epididymis is one-half that 

 of the testis proper. The weight of the testes proper, as well 

 as of accessory sexual apparatus (epididymis, seminal vesicles, et 

 cetera) , probably undergoes considerable change during cycles of 

 sexual activity. This has been noted by Disselhorst' ('98, '08) 

 in various mammals and especially in birds, for the gonads in 

 both sexes. 



My own data (table 15, fig. 7 a) indicate that the testes (in- 

 cluding the epididymis) form an average of about 0.13 per cent 

 of the body at birth, increasing gradually to a maximum of 1.50 

 per cent at ten weeks, and decreasing somewhat at later periods. 

 The variability in absolute weight is quite high, the coefficient 

 varying from 25.3 at birth to 40.7 at five months (excepting at 

 seven days, when it is only 17.7). The coefficient of variation in 

 percentage weight is much lower (10.3 to 32.8) and the correla- 

 tion with the body weight higher than might be expected (0.67 

 to 0.95, except at ten weeks, 0.48). These figures are somewhat 

 too high, however, due to 'spurious correlation.' 



