528 PERCIVAL BAILEY 



'J'his Iiokls true of all iiiMininals and is what one would expect 

 from the phylogenetic history of the plexus. His figure 42 of 

 a section anterior to the velum shows clearly the anterior extrem- 

 ity arising from tlie roof plate of the telencephalon. 



(e) The earliest appearance of the plexus in the human embryo 

 occurs in His' embryo of 13.6 mm. and his model shows clearly 

 the plexus arising just lateral to the paraphysal arch. 



(/) Hochstetter ('13) figures a section of a human embryo 

 showing the anterior extremity of the plexus, and the author 

 recently (Bailey, '15) has described three human embryos show- 

 ing the plexus extending into the roof plate alongside the para- 

 physal arch anterior to the velum transversum. 



(g) Collateral evidence is derived from the development of the 

 lateral plexus in other mammals, for example, Johnston ('09) 

 states that in the pig the plexus arises from the anterior limb of 

 the di-telencephalic groove but is separated medially from the 

 velum transversum by the paraphysal arch. 



CONCLUSIONS 



1. The lateral telencephalic plexus of Chrysemys marginata 

 arises from the anterior area chorioidea lateralis telencephali in 

 the roof plate of the telencephalon between the paraphysis and 

 the taenia fornicis of the medial hemisphere wall and in its later 

 development oversteps the taenia fornicis just anterior to the 

 point where taenia fornicis, taenia thalami, velum transversum 

 and di-telencephalic groove meet, and invaginates also the pos- 

 terior area chorioidea lateralis telencephali in the medial hemi- 

 sphere Wall, just anterior to the di-telencephalic groove. The 

 pars anterior plexus telencephali lateralis which develops from 

 the anterior area chorioidea lateralis telencephali in earlj^ stages 

 is much larger and better developed than the pars posterior 

 plexus telencephali lateralis, but in later stages develops much 

 less rapidly than the pars posterior. 



2. All the data at present available support the conclusion that 

 the lateral telencephalic plexus arises from the anterior area 

 chorioidea lateralis telencephali in the roof plate of the telen- 

 cephalon between the paraphysis (or paraphj'-sal arch) and the 



