PARAPHYSIS AND PINEAL REGION IN REPTILIA 357 



myzon, the velum is well developed and forms a curtain-like fold 

 between telencephalon and diencephalon. In amphibia it is well 

 developed at first but becomes absorbed by the excessive growth 

 of the paraphysis and plexuses. In reptiles, birds and mammals 

 it becomes proportionately less well marked especially toward 

 the median line. 



Choroid plexus 



The choroid plexuses are closely associated with the paraphysal 

 and post velar arches. The plexus choroideus lateralis springs 

 from the paraphysal arch immediately in front of and lateral to 

 the mouth of the paraphysis, fig. 25, and invaginates the dorso- 

 mesial wall of the hemisphere. In certain forms especially am- 

 phibia there is a median plexus arising from the paraphysal arch 

 immediately in front of the paraphysis and developing downwards 

 and backwards into the lower part of the third ventricle to a 

 greater or less extent, Warren (97), Necturus, figs. 9 and 11. 

 This is the plexus inferioris, anterior, or telencephalic plexus. 

 When this plexus is present the lateral plexus springs from its 

 base, Warren (97), fig. 12. In that article I used the term telen- 

 cephalic for this plexus inferioris in order to distinguish it from 

 the diencephalic plexus arising from the post velar arch. This 

 term would apply equally well to the lateral plexus as they are 

 both of telencephalic origin. I have however restricted it to 

 this lower median plexus in order to differentiate it from the upper 

 median or diencephalic plexus. This inferior or telencephalic 

 plexus does not develop in Amphioxus and appears in a very reduced 

 form in Petromyzon and Teleosts, Burckhardt (13). In Acan- 

 thias the velum projects deeply into the ventricular cavity and 

 both its surfaces become much folded to form the choroid plexus. 

 This is also the case in Accipenser and Amia and also Opsanus, 

 Terry (102). It is present in Dipnoi, Protopterus, Burckhardt 



(13). 



It is a question whether these folds of the velum represent the 

 diencephalic or telencephalic plexus of amphibia. The telen- 

 cephalic plexus, plexus inferioris, of amphibia arises from the 

 paraphysal arch in front of the paraphysis. The folds on the 



