24 CM. JACKSON 



at 20 grams, 0.29 per cent at 50 grams, 0.18 per cent at 120 grams, 

 0.15 per cent at 170 grams, 14 per cent at 200 grams, and 0.11 

 per cent at 300 grams. 



In my data grouped according to age periods (table 4, fig. 1 b) 

 the average weight of the eyeballs at birth forms about 0.52 per 

 cent of the (net) body weight in the male, and 0.54 per cent in 

 the female. The relative weight increases to a maximum at 

 seven days of about 0.60 per cent in the male, and 0.64 per cent 

 in the female.^ Thereafter the eyeballs decrease steadily in rela- 

 tive weight, excepting a slight rise at one year (which, like that 

 for the head, is probably abnormal). It will be noted that the 

 percentage weight of the eyeballs is larger in the female at every 

 age except at six weeks. Whether this expresses a true difference 

 according to sex is uncertain, however, as the difference is small 

 and the data perhaps inadequate to determine this point. 



In connection with this difference in relative weight accord- 

 ing to sex, it is noteworthy (table 4) that the absolute weight 

 of the eyeballs is nearly the same in both sexes at every age. 

 Since the average body weight of the female is smaller at each 

 of the ages noted (except six weeks), it follows that the relative 

 weight of the eyeballs must be correspondingly larger in the 

 females. 



The similarity in the absolute weight of the eyeballs in both 

 cases at various ages suggests the possibihty that the growth of 

 these organs may be somewhat independent of influences affect- 

 ing the growth of the body as a whole. This idea is to a certain 

 extent confirmed by the coefficient of variation in the absolute 

 weight of the eyeballs (table 4). Although at birth this coeffi- 

 cient (15.5) appears larger than that of the whole body (12.3), 

 after the age of seven days the variation in the absolute weight 

 of the eyeballs is exceedingly low (7.45 to 13.3), only the brain 

 approaching it in this respect. 



5 In comparing the observed with the calcuhxted values in this and the other 

 organs, it should be pointed out that even slight fluctuations in absolute weight 

 which may lie within the experimental error, may produce a much greater per- 

 centage deviation in the youngest animals, owing (1) to the small absolute size 

 of the organ, and (2) to the relatively greater weight of the organ in the earlier 

 periods. 



