200 FREDERICK TILNEY AND LUTHER F. WARREN 



The middle or post velar arch (so called by Minot 277 ) in the 

 pineal region has also been designated the Zirbelpolster by 

 Burckhardt, 47 the postparaphysis by Sorensen, 361 the dorsal sac 

 by Goronowitsch, 153 and the roof of the parencephalon by 

 Kupffer. 226 This structure, with few exceptions, forms a prom- 

 inent element of the pineal region throughout the vertebrate 

 series. In cyclostomes it is present as a simple membranous 

 sac with scant vascularity of its own, although in close approxi- 

 mation with the highly vascular mesenchyme dorsal to it. In 

 selachians it is usually somewhat more extensive yet similar in 

 its structural details. In ganoids it becomes immensely ex- 

 panded as shown by Balfour, 11 Huxley, 191 Wiedersheim, 425 

 Goronowitsch, 153 Wilder, 428 and Kingsbury. 204 Herrick 178 describes 

 the dorsal sac as a pouch lined with a single row of ependymal 

 cells with long cilia which appear to be of the epithelial, secre- 

 tory type. It is highly vascular in these fish. In teleosts, on 

 the other hand, it is not always prominent, In Opsanus, Terry 392 

 found that the dorsal sac was small and perhaps disappeared 

 altogether. In some teleosts, as in ganoids, the postvelar arch 

 is not only highly vascular, but presents ridges, secondary folds, 

 and diverticula. In amphibia, reptiles, and birds, the postvelar 

 arch becomes definitely associated with the formation of the 

 chorioid plexuses, and it does, in fact, contribute the epipthelial 

 elements to the chorioid plexus of the diencephalon. With the 

 advent of the corpus callosum in mammals the dorsal sac or 

 postvelar arch becomes somewhat overshadowed, due to the 

 introduction of the transverse commissure which lies above and 

 tends to flatten it. It, however, loses none of its tendency to 

 participate in the plexus formation, which latter in mammals 

 -attains a greater development than in many of the lower forms. 



This element of the pineal region, therefore, is to be associated 

 with the paraphyseal arch in its tendency toward specialization 

 From the lowest vertebrates upward through the phylum it 

 manifests no attempt toward the development of sensory or 

 other definitely neural elements, while the entire trend of its 

 evolution reveals a glandiferous potentiality. 



