MORPHOGENESIS OF THE CHOROID PLEXUSES 525 



DISCUSSION 



From the foregoing history and description, and from an 

 analysis of the remaining hterature, for in the history are in- 

 cluded only the most important papers and especially those 

 deaUng with the lateral telencephahc plexuses, it will be seen 

 that there are certain definite regions of the brain wall wherein 

 choroid plexuses develop. These regions and the plexuses which 

 develop from them may be tabulated as follows: 



Tela chorioidea telencephali medii— Plexus telencephali medius 

 Anterior area chorioidea lateralis telencephali — Plexus telencephali lateralis 

 (below Chelonia) 



{Anterior area chorioidea lateralis telencephali — Plexus tclencoi)hali 

 lateralis (Chelonia and above) 

 Posterior area chorioidea lateralis telencephali (heraispherici) — Plexus 

 telencephali lateralis (Chelonia and above) 

 Velum transversum — Plexus velares 

 Tela chorioidea diencephali — Plexus dicncephali 

 Tela chorioidea mcsencephali — Plexus mesenccphali 

 Tela chorioidea myelenccphali — Plexus myclencephali 



Tilney ('15) has suggested that the saccus vasculosus should 

 be reckoned with the choroid plexuses in the forms where it is 

 present and in this opinion I concur. It is best developed in 

 those forms in which the diencephalic plexus is rudimentary or 

 absent, i.e., in Cyclostomes, Selacliians and Ganoids. The plexus 

 formation is ver}' poorly de\'cloi)ed in Urodeles and never again 

 present. There should be added to the above therefore: 



Rccessus infuiuiibularis (posterior wall) — Saccus vasculosus. 



The myelencephalic ]ilexus arises in the roof of the fourth 

 ventricle, tela chorioidea nwelencephali, in every known verte- 

 brate above Amphioxus. 



The mesencephalic plexus is found only in Petromyzon, where 

 it arises from the mesencephalic roof, tela chorioidea mesen- 

 cephah. 



The diencephalic plexus arises from the tela chorioidea dien- 

 cephali. Appearing first in Cyclostomes, but very poorly 

 developed, it disappears almost entirely in Selachians, where 

 the tela chorioidea diencephali is almost completely absorbed 



