212 FREDERICK TILNEY AND LUTHER F. WARREN 



element in the complex. Both organs are well developed in 

 cyclostomes. The proximal portion of the pineal organ, al- 

 though present, is not conspicuous, but it is doubtful whether 

 any structure which may be designated a proximal portion can 

 be discerned in the parapineal organ. The stalks and end- 

 vesicles are present and highly specialized. The stalks have 

 lost their original lumina and consist of two sets of nerve fibers. 

 Both end- vesicles are well differentiated. 



In selachians the parapineal organ is entirely absent. The 

 proximal portion of the pineal organ is well marked, its stalk is 

 long and hollow, and its end- vesicle a dilated sac. 



In ganoids and teleosts the parapineal organ is rudimentary; 

 in the embryo it presents a small proximal portion which subse- 

 quently becomes much reduced in size, rendering it difficult of 

 recognition in the adult. The stalk is short and slender and con- 

 tains no lumen. The end- vesicle is very small. The proximal 

 portion of the pineal organ shows a considerable dilatation and 

 is connected, by means of a hollow stalk, with an extensive 

 end-vesicle. 



In amphibia the parapineal organ is absent. The proximal 

 portion of the pineal organ is a large, dilated sac whose lumen 

 communicates with the third ventricle. The stalk is reduced to 

 a slender nerve strand extending from the free extremity of 

 the proximal portion to the end-vesicle which lies immediately 

 beneath the skin in the region of the head. 



In the primitive reptiles, including sphenodon and lacertilia, 

 the parapineal organ is present and shows a marked development. 

 In the embryo there is a prominent proximal portion, which, 

 however, becomes gradually reduced hi size, and in the adult is 

 difficult to distinguish. The stalk of the parapineal organ 

 presents itself in the form of a long slender fasciculus of nerve 

 fibers which connects the superior commissure with an end- 

 vesicle. The pineal organ shows a highly specialized proximal 

 portion which is large and convoluted. Its cavity communicates 

 with the third ventricle through a narrow canal. The stalk is 

 short and contains a cavity which communicates with the end- 

 vesicle at its distal extremity and also with the proximal por- 



