80 FREDERICK TILNEY AND LUTHER F. WARREN 



in the selachians. It makes a somewhat abortive appearance in 

 the ganoids and teleosts. In urodeles and anura it disappears 

 altogether but when again it does occur as a feature of the 

 epiphyseal complex, it has assumed such proportions as to 

 make it by far the most prominent structure in this area of the 

 brain. In the prosaurians and the saurians, it is a most con- 

 spicuous element. As may easily be presumed, the pineal stalk 

 and its analogue, the parapineal stalk, follow very closely the 

 frequency of occurrence of the two end-vesicles. Thus the 

 pineal stalk is present in cyclostomes, selachians, ganoids, 

 teleosts, urodeles, anura, prosaurians and saurians, but disap- 

 pears in the higher forms. The parapineal stalk is present in 

 the cyclostomes, but does not appear in selachians. It has an 

 abortive form in ganoids and teleosts, is absent in urodeles and 

 anura, occurs in its most marked representation in prosaurians 

 and saurians, and thereafter disappears altogether. 



6. THE COMPARATIVE ANATOMY AND HISTOLOGY OF THE 

 EPIPHYSEAL COMPLEX 



In the light of the embryological development of the epiphy- 

 seal complex, the difficulties in the adult morphology of these 

 organs are much diminished. The following description will 

 deal with the comparative anatomy and histology of the two 

 epiphyseal elements in the different classes of vertebrates and 

 will be based upon the observations of the different species 

 already investigated. 



1. The comparative anatomy and histology of the epiphyseal 

 complex in cyclostomes 



The pineal organ in cyclostomes presents the three charac- 

 teristic parts, namely, a proximal portion, a stalk, and an end- 

 vesicle. Each of these is more or less highly specialized. The 

 end-vesicle has the form of a small elliptical vesicle. In its longest 

 diameter cephalocaudad, it is 0.35 mm. in length. This measure- 

 ment was made in Petromyzon by Studnicka. 384 It presents 



