14 Weight of the Central Nervous System of the Frog 



matured, in the sense that these latter have sent out both kinds of branches, dendrites 

 and axone, and that the axone has acquired a medullary sheath. 



As the frog grows larger the nervous system increases in weight. Concerning 

 the method of this increase the following statements can be made: First, there is no 

 cell division in either the supporting tissues or the nervous tissues at this time ; hence 

 the increase in weight is due to the enlargement of cell elements which are already 

 present in the system. In the case of the neurones already developed and function- 

 ally active, this enlargement means an increase in the volume of the cell bodies, in 

 the number and size of the dendrites, and in the length and diameter of the axone and 

 its medullary sheath. In the case of the undeveloped neurones, it means a rather 

 rapid acquisition of the branches and medullary sheath, to be followed by the slower 

 changes just described above. In general, these changes tend to increase the com- 

 plexity of the entire system, but, so far as they represent a mere lengthening of the 

 connecting axones and a mere increase in their diameter, the added weight does not 

 necessarily imply the increase in complexity, but only a passive adaptation of the 

 system to the increasing size of the cavities in which it is contained. 



The formula which we have employed indicates that where the body-weight is 

 expressed by numbers increasing in geometrical progression, the weight of the central 

 nervous system is expressed by numbers increasing in arithmetical progression, these 

 latter being multiplied by a factor derived from the length of the entire animal. 



Certainly this factor, depending on the length of the frog, is to be associated 

 with the increase in the length of the brain and cord, but no satisfactory interrelation 

 between this factor and this part of the growth process has been established. We are 

 compelled, therefore, at this time to be content with pointing out the entire series of 

 events which the formula expresses, without attempting to correlate any portion of the 

 formula with any special part of the growth change. 



SUMMARY 



The formulsB here presented apply to the two species of frog: R. cateshiana, the 

 bullfrog, and R. virescens, the leopard frog. The best results are obtained when 

 frogs taken in midsummer (months of July and August) are alone used. The frog 

 must be in normal condition and have a body-weight of 5 grams or more. When 

 these conditions are fulfilled, then the weight in milligrams of the central nervous 

 system [C.N.S.) of the bullfrog can be determined with a high degree of accuracy by 

 the formula 



C.N.S. = (Log W X f/L) 30, 



where W is the weight of the frog in grams, L the entire length in millimeters, and 

 30 a constant peculiar to the species. 



In the same way the weight of the central nervous system [C.N.S.) in milligrams 

 can be determined for the leopard frog by the formula 



28 



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