MORPHOLOGY OF THE FOREBRAIN 439 



massive wall of the hemisphere by the taenia fornicis which is 

 directly continuous caudad with the taenia thalami. From this 

 roof membrane the paraphysis and plexus chorioideus of the 

 third ventricle are developed. The frog larva figured by Kupffer 

 ('06) is apparently older than the specimens here figured and 

 resembles the adult frog more closely. 



The four fundamental parts of the wall of the hemisphere of 

 the adult frog are here imperfectly separate, as in the urodeles. 

 The cells of the dorso-median part have the diffuse arrangement 

 characteristic of this region in the adults of both the frog and 

 urodeles. The relations of the commissura pallii anterior to this 

 region (fig. 32, commis. hippocampi) fix its homology as prinior- 

 dium hippocampi. In the region of the lamina terminalis and 

 farther rostrad the primordium hippocampi is clearly separated 

 from the precommissural body by a cell-free zona limitans (figs. 

 29 and 30). An ependymal sulcus in fig. 29 marks the position 

 of the fissura limitans hippocampi of the adult, a fissure which 

 is termed fissura arcuata by Gaupp, though incorrectly,, as will 

 appear beyond, for the latter fissure is absent in both larval and 

 adult Amphibia. 



The relations at the posterior pole are very similar in these 

 larvae to those of Amblystoma of 30 mm. to 40 mm., described 

 above. The lateral ventricle is extended into it for a short dis- 

 tance caudad of the lamina terminalis and the primordium hip- 

 pocampi tissue is also extended farther caudad. The commissura 

 pallii anterior is derived chiefly from the primordium hippocampi, 

 but partly from the posterior pole, and at the point where it 

 turns downward from the latter to pass behind the interventricu- 

 lar foramen into the lamina terminalis it comes in contact with 

 the stria medullaris, as in urodeles. At this point of contact there 

 is developed a small dense nucleus, which here bears the same 

 relation to the taenia fornicis as does the caudal end of the pars 

 fimbrialis septi farther rostrad (cf. figs. 31 and 32). These nuclei 

 are, however, not continuous and are of quite different origins. 

 This nucleus is termed by Gaupp in the adult frog nucleus supra- 

 commissuralis. I term it for reasons which will appear beyond 

 the nucleus of the commissura hippocampi. In the tadpole it 



