148 C. M. Jackson. 



that the brain is relatively larger in the embryo or newborn than in 

 the adult. 



For the spinal cord, however, this rule is apparently not constant. 

 In the chick and stickleback, the spinal cord in the adult diminishes 

 in relative size ; but in the shrew and (to a slight extent) in the dog, 

 it apparently increases from newborn to adult. 



The eyeballs are in all the animals just mentioned (excepting the 

 salamander?) relatively smaller in the adult. In the chick embryo 

 of the 11th day of incubation (Welcker and Brandt), the eyeballs 

 form nearly 25 per cent of the entire body. In the newborn chick, 

 they have decreased to about 3 per cent, and in the adult to .3 per 

 cent or A per cent. 



The thyroid and thymus glands in the dog and shrew appear to 

 remain of about the same relative weight in the adult as in the new- 

 born. For the spleen, this applies to the chick as well as to the dog 

 and shrew. 



The heart appears relatively smaller in the adult stickleback, chick 

 and dog; but larger in the salamander and shrew ( ?). In the chick, 

 dog and shrew, the lungs are relatively smaller in the adult than in 

 the embryo or newborn. 



The alimentary canal is relatively larger in the adult stickleback, 

 dog and shrew ; but smaller in the salamander /md chick. The liver 

 is relatively much smaller in the adult shrew, slightly smaller in the 

 adult stickleback, chick and dog; but much larger in the adult sala- 

 mander than in the embryo. 



The kidneys appear relatively larger in the adult stickleback and 

 salamander, but smaller in the shrew, dog and chick (slightly). The 

 suprarenal glands are relatively slightly larger in the adult shrew, 

 (no data on other forms). The reproductive glands are relatively 

 larger in the adult chick, but smaller in the dog and shrew. 



If we compare the foregoing data with the course of growth in the 

 human body, two facts stand out clearly: In the first place, it is 

 evident that, although the gro^vth rate of the body as a whole varies 

 greatly in different animals, it is greatest in the early embryo (at 

 least in birds and mammals). In the second place, it is evident that 

 ill vertebrates in general the prenatal growth is relatively greater 



