420 HISTORY OF THE HUMAN BODY 



ones already mentioned (tcenice chorioides), which pass 

 through the interventricular foramina and supply the two 

 lateral ventricles of the telencephalon. 



Behind the plexuses there appear in the mid-dorsal line typi- 

 cally two median diverticula, which, owing to the many grades 

 of development under which they appear, as well as to the fact 

 that they have long been treated as identical, have received a 

 large number of distinct designations. The more anterior of 

 these is best known as the paraphysis, the posterior one the 

 epiphysis, but the former is also correctly known as the parietal 

 organ, the latter as the pineal organ. Both show a tendency 

 to pass through the skull and attain a position directly beneath 

 the skin in the middle line, developing there a rudimentary 

 sense organ of uncertain nature, but probably an eye in each 

 case. 



In the cyclostome Petromyzon, both structures attain consid- 

 erable development, and the optical structure of the epiphysial 

 organ is evident through the occurrence of pigment in what 

 may be well a vestigial retina. The paraphysial organ is 

 smaller, but of similar structure. In no other form are both 

 of these structures so well developed, but in several cases one 

 may attain an even higher development while the other is rudi- 

 mentary. In some instances the highest point in development 

 is reached during embryonic life, while in others it is exhibited 

 by the adult. Thus in the selachians, the epiphysis passes 

 through a minute foramen in the skull and reaches the surface ; 

 its terminal organ is visible externally, but the paraphysis is 

 not developed at all. In frogs and toads the paraphysis attains 

 a development similar to that of the epiphysis in the former 

 case, while this latter part has not been found. The para- 

 physial organ, here known as the " frontal organ," is plainly 

 visible externally, but in the adult is entirely separated from 

 the brain by the retrogression of its stalk. The highest de- 

 velopment of either organ is reached among certain lizards, 

 where it is the epiphysis that is thus favored. The terminal 

 organ here lies in a socket (parietal foramen) formed in the 

 interparietal suture and represents a fairly good eye, with pig- 



