192 'Journal of Comparative Neurology and Psychology. 



of them as was needed, were fixed in i per cent osmic acid, and 

 then embedded and sectioned in the usual manner. The sec- 

 tions were made 12// in thickness, and at the knee, the number 

 of medullated fibers in the trunks of the nervus tibiahs and nervus 

 peroneous was counted. This number was contrasted with that 

 found in the four trunks entering the foot, namely: the ramus 

 superficialis and ramus profundus of the nervus tibialis, and the 

 nervus peroneus lateralis and medialis. 



At both levels the number of fibers in several successive sections 

 was counted, and the average taken. The results of this exam- 

 ination are presented in Table 17. 



TABLE 17 . 



Showing the number of medullated fibers at the level of the knee and ankle in the 

 leg of the tadpole. Tadpoles of Rana pipiens. 



It appears from this that both the absolute and relative number 

 of medullated fibers entering the foot, increases as the leg of the 

 tadpole becomes longer, rising from i : 5.75 to i : 2.42. In a 

 large mature frog. Dr. Dunn ('02) has shown that the ratio is 

 I : 1.66, an increase of 3I fold over the ratio in the smallest tad- 

 pole. At the moment however we have no data by which to 

 determine when the ratio found in the largest tadpole's leg here 

 examined passes over to that in the mature frog. 



It is assumed that the medullated fibers which are counted at 

 he level of the knee, represent fibers already medullated through- 

 out their entire length, as well as fibers incompletely medullated, 

 but having a sheath extending as far as the level of the section. 

 The same assumption is made for the fibers at the level of the 

 foot. 



