MORPHOLOGY OF THE FOREBRATN 447 



lamina terminalis, the other is dorsal in the commissura superior. 

 The configuration is such that the primordium hippocampi touches 

 the anterior border of the thalamus and, as in the larva, these 

 two commissural tracts converge at the junction of the taenia 

 fornicis with the taenia thalami. 



The relations here are similar to those of urodeles and frog tad- 

 poles (see pp. 426 and 439), except for the great increase in size of 

 the supraforaminal part of the precommissural body, or pars 

 fimbrialis septi, and the reduction of the eminentia thalami to 

 form the small but dense nucleus of the hippocampal commissure, 

 which lies at the junction of the fimbria fiber complex with the 

 stria medullaris ventrally of the pars fimbrialis septi. The sum- 

 mary of fiber connections at this point given for urodeles on p. 

 427 applies also to the frog. 



The posterior pole of the hemisphere is that portion of the two 

 dorsal parts which projects caudad beyond the level of the lamina 

 terminalis; it is the parathalamic brain. Here the dorso-lateral 

 and dorso-medial parts come together as in urodeles and in some 

 measure lose their distinctive characteristics, suggesting the un- 

 differentiated condition of the so-called lobus hippocampi or 

 nucleus pyriformis (Sheldon) of teleosts (see p. 451). It receives 

 numerous secondaiy olfactory fibers from the olfactory bulb and 

 gives rise to the tractus cortico-habenularis lateralis (tractus 

 taeniae, Edinger) and the commissura pallii posterior, as has been 

 already suggested by Elliot Smith ('03, p. 495). 



The hemispheres of the adult frog are so much farther evag- 

 inated, as compared with the larvae and the urodeles, as to make 

 correlation with reptiles and higher forms relatively easy. The 

 dorsal parts are clearly separated from the ventral parts for the 

 entire length of the hemisphere both by the external and internal 

 zona limitans and by total fissures; and the ventral parts alone 

 are in direct massive contact with the corresponding parts of the 

 diencephalon. The dorso-median part is in contact with the 

 ventral part of the thalamus, but not with any part of the thal- 

 amus dorsally of the sulcus diencephalicus medius. This con- 

 tact is probably secondarily acquired in Amphibia (see p. 420) 

 and it does not occur in Amniota. 



