128 LAWSON G. LOWREY 



cent of the total. In the adult they form only 0.011 per cent of 

 the total weight. 



From a few scattering observations on the human eyeballs, 

 it seems that they are relatively smaller than those of the pig in 

 the early stages ; of about the same relative size in the later stages 

 of prenatal life; relatively smaller at birth, and about twice as 

 large relatively in the adult. 



Welcker and Brandt record some observations on the chick 

 embryo. At the eleventh day of incubation, the eyeballs form 

 nearly 25 per cent of the total body. At birth they form about 

 3 per cent, and in the adult about 0.3 per cent. 



Thyroid gland. {Table 1: jig. 5). The thyroid makes its first 

 appearance (being too small for acclirate dissecting earlier) in 

 the 58 mm. litter, when it has its maximum observed relative 

 size of . 045 per cent of the entire body weight. From then on 

 until the end of the prenatal period, it averages from 0.02 per 

 cent to 0.03 per cent, forming 0.026 per cent, at birth. In the 

 adult it forms only 0.004 per cent of the total. The individual 

 measurements are rather variable for this gland, but it averages 

 very regularly throughout. 



The human thyroid, forming . 035 per cent of the total embryo 

 at two months, increases to . 11 per cent or . 125 per cent (still- 

 born or live-born). In the adult, it forms 0.05 per cent of the 

 total according to Vierordt. It is, therefore, at all stages, a 

 relatively larger and probably more important organ in the human 

 than in the pig. 



Thymus. {Table 1: fig. 4)- The thymus is liable to a consid- 

 erable amount of individual variation, and shows a gradual in- 

 crease in relative size throughout prenatal life. Where first 

 measured, at 58 mm., it forms practically 0.1 per cent of the total 

 weight. Near full term, it forms 0.37 per cent of the total. 

 It was not found in the adult, though it may possibly have been 

 overlooked. 



In the human, the thymus forms 0.008 per cent of the entire 

 body at the end of the second fetal month ; and about . 3 per cent 

 at birth. Vierordt gives the adult size as 0.04 per cent of the 

 total weight. 



