396 DIFFERENTIATION OF AMPHIBIAN NERVOUS SYSTEM 



provided by grafting experiments^ in which portions of neural fold 

 from the head-region of the early neurula were grafted into the 

 ventral epidermis of other neurulae, and there produced cartilage 

 as well as nerve-cord and ganglia. Grafts of the corresponding 

 presumptive region of the late gastrula only produced nerve cord 

 and ganglia : it would appear that the capacity to produce cartilage 

 is determined later than that to produce neural elements. From 

 other experiments, it appears that neural crest tissue has the power 

 of determining other tissue (e.g. presumptive epidermis) to dif- 

 ferentiate into cartilage" (see p. 193), and this might be taken as a 

 case of homoiogenetic induction. 



Experiments on heteroplastic grafts of axolotl tissues into 

 Triton hosts have shown that the neural crest cells in the trunk- 

 region also may have various prospective fates. While some of 

 them give rise to the trunk spinal ganglia, others migrate in the 

 form of mesenchyme to the outer side of the myotomes, and 

 into the dorsal and ventral fins.^ 



Further differentiations of the nervous system may occur under 

 the influence of hormones. Strictly speaking, such cases fall 

 beyond the scope of this book. But we may mention the well-known 

 fact that human cerebral development is incomplete without the 

 presence of a sufficiency of thyroid hormone. Another case of brain 

 differentiation under the influence of thyroid is seen in Amphibia. 

 Here a marked change in the proportions and shape of the parts of 

 the brain occur at metamorphosis. Thyroidectomised tadpoles 

 preserve in the main the larval type of brain.* Further, the 

 morphogenetic changes occurring in the amphibian brain at 

 metamorphosis are known to be accompanied by psychological 

 changes. Salamander larvae can be tamed and trained to take 

 food out of the human hand; but this habit vanishes completely 

 from the day of metamorphosis.^ This 'forgetting is clearly 

 due, not to a psychological process of suppression ' (as suggested 

 by W. H. Rivers in his Instinct a?td the Unconscious, 1920), but 

 to morphological changes in the nervous system. 



^ Raven, 1933 a. ^ Holtfreter, 1933 b. ^ Raven, 193 1 b. 



* B. M. Allen, 1924. ^ Flower, 1927. 



