42 FREDERICK TILNEY AND LUTHER F. WARREN 



inception this evagination is a small, cordiform enlargement 

 which rests at first directly upon the ectoderm of the upper 

 surface of the head. The gradual lengthening of the tubular 

 pineal organ, which is similar to what occurs in Petromyzon, is 

 in the main due to the fact that a very large amount of mesen- 

 chyme makes its appearance between the roof of the brain and 

 the under surface of the skull. In this way the end-vesicle of 

 the pineal organ maintains its relative position to the ectoderm 

 and thus always remains near the surface of the skin. In many 

 instances the end-vesicle comes to lie in a large foramen of the 

 skull, that is, the parietal foramen which makes its appearance 

 at a later stage of development.. 



Considering the embryological development of the pineal 

 region in selachians, Locy 244 holds that two pairs of accessory 

 optic vesicles are preserved in the cephalic plate of Elasmo- 

 branchs, his particular reference being to Squalus acanthias. 

 These accessory optic vesicles together with the primary optic 

 vesicles give rise to two pairs of rudimentary eyes. The epi- 

 physis is, therefore, of double origin, forming a united pair of 

 accessory optic vesicles, and since the latter are homologous 

 with the lateral eyes, the epiphysis itself must be homologous 

 with these eyes also. His contention that the pineal outgrowths 

 arise from two pairs of vesicles that are homologous with those 

 giving origin to the lateral eyes has not been altogether sustained 

 by other observers. Locy is also of the opinion that it is highly 

 probable that the enlarged distal end of the epiphysis in Squalus 

 is homologous with the pineal eye in those forms in which it is 

 differentiated. Goette 152 in 1875 expressed the opinion that the 

 epiphysis in selachians was a product of differentiation at the 

 point of union between the brain and the epidermis. He com- 

 pares the pineal organ to the pore which persists for a long time 

 in the embryo of Amphioxus and leads into the encephalic cavi- 

 ties. Ehlers 108 in 1878 confirmed the findings of Balfour in 

 Raia clavata and Acanthias vulgaris. An interesting observa- 

 tion in this connection is the finding by Cattie 60 of the pineal 

 organ in Torpedo marmorata. Cattie observed the organ in the 

 embryonic state in this form. The importance of this observa- 



