526 PERCIVAL BAILEY 



by. the overgrowth of the vekim transversum. In Ganoids, the 

 tela chorioidea diencephaU begins to emerge, forming a thin 

 walled sac, and in Urodeles is again invaginated by an enormous 

 diencephalic plexus. It may be that the diencephalic plexus 

 in Selachians and Ganoids is represented by the choroidal folds 

 on the posterior limb of the velum, developing from the anterior 

 portion of the tela chorioidea diencephali which has been drawn 

 down into the velum transversum by the overgrowth of the 

 latter. The diencephahc plexus is present in all forms above 

 Urodeles but is never again so well developed. 



The velar plexuses are present only in Selachians, Ganoids, 

 and Urodeles. They may involve either the diencephalic limb 

 or telencephalic limb, or the entire velum transversum. The 

 choroidal folds on the diencephalic limb have been homologized, 

 in Selachians and Ganoids, with the diencephalic plexus of other 

 forms, and with some reason. The choroidal folds on the ante- 

 rior limb are not homologous with either the median or lateral 

 telencephalic plexuses. 



The median telencephalic plexus arises from the tela chorioidia 

 telencephali medii, just in front of the paraphysis (paraphysal 

 arch of Mammalia), from the Selachians to the Chelonia, in- 

 clusive, with the apparent exception of the Ganoids. It is 

 not constantly present in Chelonia and is never found in Mam- 

 malia. Its development seems to be in inverse ratio to the 

 degree of development of the lateral telencephalic plexus and 

 the latter in direct ratio to the size of the hemispheres. 



The lateral telencephalic plexus is found in all groups of verte- 

 brates from the lowest to the highest in the line of ascent of 

 Mammals with the exception of the very lowest, Amphioxus 

 and the Cyclostomes. Apparently sporadically and imperfectly 

 developed in Selachians and Ganoids, it is present constantly 

 thereafter. We have stated previously that it is absent in Teleosts 

 and Anura, with which we are not concerned. In all forms below 

 Chelonia, it develops in what I have called elsewhere (Bailey, '15) 

 the anterior lateral telencephalic chorioidal area, in the roof 

 plate of the telencephalon between the paraphysis and the taenia 

 fornicis of the medial hemisphere wall. Where the median 



