DEVELOPMENT OF NERVE ELEMENTS. 



I6 7 



five frogs which were intermediate between these ex- 

 tremes, it is found that from the smallest frog up to the 

 heaviest each additional gramme of frog is accompanied 

 by an absolute increase of about 50 nerve fibres in the 

 ventral roots. The following table exhibits the results 

 in greater detail. 



TABLE 38. SHOWING THE NUMBER OF FIBRES IN THE VENTRAL 

 ROOTS OF FROGS OF THE SAME SPECIES, RANGED ACCORDING 

 TO THEIR INCREASE IN BODY-WEIGHT. (Birge.} 



. From these observations it is concluded that the rate 

 at which the nerve fibres are produced in a frog before 

 it reaches 1-5 grammes in weight, is many times that at 

 which the production takes place in the larger frogs. It 

 is certainly remarkable that after this earliest stage the 

 rate of increase is quite constant. An examination of 

 the figures in detail shows that the increase in the num- 

 ber of fibres is greatest and most persistent in the cervi- 

 cal group (Roots L, II., III.), next in the lumbar group 

 (Roots VIII., IX., X.), while in both cases the number of 

 fibres in the largest frog is more than double that in the 

 smallest, whereas in the thoracic region (Roots IV., V., 

 VI., VII.) the corresponding ratio is only 1-1*3. To 

 guard against a too hasty interpretation of these results, 



