78 FREDERICK TILNEY AND LUTHER F. WARREN 



The diencephalic choroid plexus and lateral telencephalic plexuses 

 are well marked and develop essentially as described in other verte- 

 brates. There is no trace of the median telencephalic plexus so notice- 

 able in Amphibia. 



The epiphysis forms a short hollow stalk with thick walls and in- 

 clined slightly backward over the posterior commissure. 



The superior and posterior commissures are formed as in other 

 vertebrates. The posterior commissure is characterized by its pre- 

 cocious development and by the extent that it invades the pars inter- 

 calaris of the forebrain in early embryos (fig. 44). 



It will be observed that in the ontogenesis of each element in 

 the epiphyseal complex, three distinct parts may be discerned in 

 each of the two organs entering into it. Thus, the pineal organ 

 may have an end-sac, a stalk, and a proximal portion, and the 

 same is true of the parapineal organ. Considered in the light 

 of comparative embryology, it will be seen that the most con- 

 stant part throughout the phylum is the proximal portion of 

 the pineal organ. This, beginning with a moderate prominence, 

 as in the cyclostomes, rises to a very prominent element in sela- 

 chians and maintains this prominence with somewhat of an 

 increase in its importance throughout the entire series, with the 

 single exception of crocodilia, in which the pineal body is said 

 by Sorensen 363 to be entirely wanting. On the other hand, the 

 proximal portion of the parapineal organ shows a strikingly low 

 percentage of occurrence throughout the phylum. It may 

 perhaps be accredited to the cyclostomes, if one takes into 

 account the thickened portion of the unusually large commissura 

 habenularis, but thereafter in the series it seems to disappear 

 entirely. 



The next most constant structure in the epiphyseal complex 

 is the end- vesicle of the pineal organ. This maintains a high 

 degree of prominence in cyclostomes, selachians, ganoids, teleosts, 

 urodeles and anura. It shows a conspicuous tendency to atten- 

 uate in the prosaurians and saurians and finally in the ophidians, 

 and in all the orders thereafter it is notable for its absence. The 

 analogue of the pineal end-vesicle, namely, the parapineal end- 

 vesicle, is much more irregular in its occurrence throughout the 

 phylum, but on the other hand, in certain forms it presents such 



