Allen, Association in the Guinea Pig. 353 



It will be seen that the percentage increase in both the 

 gray and white substance is much greater at the lumbar level 

 than at the levels above, the lumbar cord, therefore, showing 

 the greatest amount of developmental change from birth to 

 maturity. 



The progress of meduUation in the thirty day guinea pig 

 has attained about the stage reached by a thirty-five day white 

 rat. The table below compares the areas of the spinal cords 

 of the two animals. 



TABLE III. 



Table showing the areas of the cros.« sections of the spinal cord.s of a 

 guinea pig thirty days old, and of a white rat thirty-five day.s old. 



It will be seen that the absolute areas are closely compara- 

 ble, the cervical cord of the white rat being, however, larger 

 than that of the guinea pig ; so that in volume of the spinal 

 cord as well as in the stage of meduUation the guinea pig of 

 thirty days ma^' be compared with the white rat of thirty-five 

 days. 



To repeat what has been said before, by way of summary, 

 the spinal cord of the guinea pig increases in area by the devel- 

 opment of new fibers, and by the growth of fibers already 

 formed at birth. When we see how heavily medullated the 

 adult cord is. and know how much it has increased in area we 

 conclude that there has been a very great addition of substance 

 by both of these methods. 



///. The EncepJialon of the Guinea Pig. 

 Cerebrum. 

 In the encephalon at different ages the relation of the parts 

 is not the same, so that it is impossible to get exactly compar- 

 able sections, and structures in the optic thalami were chosen 



[61] 



