MORPHOLOGY OF THE FOREBRAIN 441 



tially as described for Amblystoma (p. 424 ff.). Associated with 

 the stria medullaris, are commissural fibers between the posterior 

 poles of the hemispheres running by way of the commissura supe- 

 rior. This is the commissura pallii posterior. 



On account of the absence of the diencephalic flexure in these 

 brains the configuration of the thalamic structures bordering the 

 telencephalon is very different from that of larval urodeles, but 

 is the same in principle. The median and lateral ventral parts 

 of the hemisphere pass back into the nucleus preopticus and pro- 

 minentia fascicularis as before. The whole of the large nucleus 

 preopticus, which belongs to the unevaginated forebrain or telen- 

 cephalon medium, is overlapped dorsally by diencephalic struc- 

 tures, the sulcus diencephalicus ventralis forming the boundary 

 between them (see figs. 33 to 38). As in the adult frog, this 

 nucleus is very large and shows considerable internal differentia- 

 tion. Correlated with the straightening out of the diencephalic 

 flexure and the further evagination of the cerebral hemispheres 

 in the anuran larvae we find, as mentioned above, that the emi- 

 nentia thalami at the caudal border of the interventricular fora- 

 men is reduced to a small vestige, the nucleus of the hippocampal 

 commissure (figs. 32 and 33). 



Immediately caudal to the foramen the pars ventralis thalami 

 enlarges to assume the same relations as in urodeles, being bounded 

 by the sulcus medius and sulcus ventralis. 



The ventro-lateral part is extended for a considerable distance 

 behind the posterior pole of the hemisphere into the unevaginated 

 telencephalon medium dorso-laterally of the preoptic nucleus 

 (figs. 33, 34, 35). This tissue, including a part of the primordial 

 corpus striatum, is in the course of further development almost 

 all evaginated into the hemisphere proper, as shown by the adult 

 configuration. 



The habenula and superior commissure are placed far forward, 

 as in urodeles, and the pars dorsalis thalami lies almost wholly 

 caudal to these structures. The two segments of the sulcus 

 diencephalicus dorsalis separate the epithalamus from the thala- 

 mus as described on p. 431 for urodeles. 



The corpus geniculatum laterale appears near the rostral end 



