THE PINEAL BODY 199 



From the remainder of the paraphyseal arch there develop 

 in many classes of vertebrates several chorioidal processes. In 

 cyclostomes, selachians, teleosts, and ganoids, two such plexuses, 

 more or less well developed, may be recognized, namely, the 

 lateral and inferior telencephalic chorioid plexuses. The inferior 

 chorioid plexus attains its most marked proportions in amphibia, 

 while in all of the higher vertebrates its prominence declines. 

 This is likewise true of the lateral chorioid plexus. Histologi- 

 cally and topographically, the significance of these plexuses is 

 not difficult to discern; their rich vascularization, their tendency 

 toward glomerular arrangement together with the relations and 

 modifications of the ependymal cells which enter into them 

 leave little room to doubt that they are glandular in nature. 

 Indeed, the present tendency is to refer to these structures as 

 chorioidal glands, thus deputing to them a definite, secretory 

 function in relation to the cerebrospinal fluid. Even the older 

 conceptions of the chorioid plexuses recognized this physio- 

 logical possibility in connection with the plexuses. 



The morphological fact concerning the first and most cephalic 

 of the three arches in the pineal region discloses a predominant 

 tendency for its derivatives to give rise to glandular structures, 

 while, on the other hand, there is no evidence that it has ever 

 been engaged in definite neural mechanisms. 



The structure which forms the boundary between the prevelar 

 or paraphyseal arch and the postvelar arch is the velum trans - 

 versum. Like the paraphyseal arch, it attains its greatest con- 

 spicuity in the lower vertebrates and in the higher forms becomes 

 less prominent. In mammals its appearance is most pronounced 

 in the embryonic period from which time it becomes progres- 

 sively reduced, being present in the adults of most orders as a 

 more or less well-marked rudiment. In most classes of verte- 

 brates it becomes associated with a dense mesenchymatous in- 

 vasion which results in a fairly rich vascularization. This com- 

 bination of ependymal cells and blood vessels often takes the form 

 of a plexus, and when such is the case the velum transversum 

 aligns itself with the structures derived from the paraphyseal 

 arch in the absence of any definitely neural elements and the 

 tendency toward glandular formation. 



