208 H. SAXTON BITRK 



On thv other hand Harrison ('10) has shown that embryonic- 

 nerve cells taken from the medullary cord of frog larvae will 

 produce protoplasmic nerve processes when cultivated in vitro, 

 but it is manifestly impossible to determine by such methods 

 the extent to which development will proceed without the inter- 

 vention of functional activity. As has been shown elsewhere 

 (Burr '16), development and differentiation of the central ner- 

 vous system will progress to a certain point without the presence 

 of a functioning end organ. Beyond this point functional 

 activity is essential for continued growth. In the experiments 

 just referred to, the telencephalon was quite completely organ- 

 ized without the ingrowth of the olfactory fibers, but this nerve 

 is not the only one whose fibers penetrate the hemisphere, for 

 Herrick ('10) has shown that in the posterior third of the telen- 

 cephalon of Amblystoma there are at least two centers that are 

 directly connected with the hypothalamus and the pars dorsalis 

 thalami by ascending projection fibers. Hence it seems probable 

 that the apparent self-differentiation of the telencephalon may 

 be due in some degree to the presence of the forward growing 

 nerve fibers which establish this connection, the functional stim- 

 ulus imparted by them being sufficient to carry differentiation 

 some distance. Experimental evidence that is in favor of this 

 view is seen in the fact that in the posterior margin of the cur- 

 tain which develops across the foramen of Monro on the re- 

 moval of the telencephalon and nasal placode, there appears in 

 the older larvae a rounded mass of nerve cells and fibers. This 

 mass is in direct communication with the nucleus habenulae 

 and the hypothalamus, a tract of fibers reaching it from each 

 of these regions. Evidently the neuroblasts at the posterior 

 margin of the interventricular foramen have been stimulated 

 to further development by the ingrowth of axones from lower 

 centers. This mass is, then, in all probability the rudiment 

 of the primitive pallium. 



The second series of experiments involved the removal of the 

 right hemisphere without the removal of the nasal placode. 

 The results here are definite and conclusive. A curtain of epen- 

 dymal cells is formed across the foramen of Monro as in the 



