28 HAROLD SAXTON BURR 



two periods, in the first of which growth and differentiation was 

 independent of functional activity, but in the second of which 

 functional activity determined whether or not growth would con- 

 tinue. But as Forel had previously pointed out, the atrophy 

 which made the distinction in size of the cells evident was an 

 exceedingly slow process as evidenced by the fact that this dis- 

 tinction did not occur until just before metamorphosis. 



Two other investigators may be mentioned here, who have 

 worked along this line. In 1909 Shorey performed a similar 

 series of experiments on chick and amphibian embryos. She 

 found in the chick an almost immediate effect of the destruction 

 of the limb bud on the size of the ventral horn at the limb level. 

 But whereas Braus concluded that the neurones were self-differ- 

 entiating and that the reduction in size was a secondary one due 

 to the absence of function, Shorey argued that the neurones did 

 not differentiate without the stimulus of function and environ- 

 ment. 



So far as it has been possible to ascertain, DiJrken ('11) is the 

 only investigator who has attempted to carry this question further 

 to a consideration of the effect of the absence of an end organ on 

 the gross morphology of the brain. He found very extensive 

 changes in the shape and organization of the brain of Rana fusca 

 and R. esculenta when the fore or hind limbs were extirpated at 

 an early age. The abnormalities affected not only the somatic 

 motor areas, but also parts of the diencephalon and telencephalon, 

 notably in the roof. As will be seen, the experiments with which 

 the present report is concerned show no such fundamental 

 upheavals. 



The last two named investigators, while using embryonic 

 material, worked with individuals in which the peripheral nervous 

 system had already partly developed, and it is suggested that the 

 fundamental discrepancy between their results and those that 

 follow may be due to this factor. 



From the experiments of Braus it is evident that there is a 

 possibility that Gudden's atrophy, when it can be induced, would 

 make possible the tracing of a given system of nerve fibers from 

 its peripheral ending to its cortical origin with all its collateral 



