Humphrey, Brain of the Snapping Turtle. 97 



case of the origin of the parietal eye from the epiphysis the origin 

 of the nerve outside of the epiphysial stalk would vitiate the 

 homology with the lateral eyes. If the homology is strictly 

 true the stalk of the epiphysis would stand in the relation of 

 the optic nerve. For the nerve to grow from the brain into the 

 eye is a manner of development not found elsewhere in any 

 sensory organ. In a recent article Locy (55) has shown in the 

 early embryos of Squalus acanthias, the existence of three pairs 

 of optic vesicles on the neural plate. The cephalic pair of these 

 vesicles develops into the lateral eyes, the third pair disappears, 

 while the middle pair passes into the diencephal, blends across 

 the meson and develops from their dorsal edge the epiphysial 

 outgrowth. This accords with the theory of homology with 

 the lateral eyes, showing as it does the earliest origin of these 

 organs to be exactly similar. Further investigation of this 

 phase of development of the embryos of different forms will be 

 awaited with interest, in the hope that it may aid in making 

 clear the intricacies of this perplexing region and if possible 

 bring into harmony the conflicting results of different observers. 

 No material being at hand for a study of the early devel- 

 opment of the epiphysis, nothing has been attempted but to 

 describe this organ and its relations in the late embryo and in 

 the adult. The study of the embryos gave the most satisfac- 

 tory results, both because of the manner of sectioning and the 

 relative simplicity of the structure of the epiphysis and the 

 surrounding organs. The pineal eye is not present in Chely- 

 dra. The epiphysis rises immediately caudad of the supracom- 

 missure and is projected cephalad in a curved course from its 

 origin (Fig. 30). Its structure of columnar deeply staining 

 endymal cells based upon a layer of connective tissue — pia, 

 enables the observer to easily differentiate it from the surround- 

 ing structures. In form it is a slightly expanded sac, joined to 

 the diencephal by a narrow stalk. This stalk becomes greatly 

 lengthened and convoluted at its base in the adult. In the em- 

 bryo of the advanced stage the cavity of the epiphysis is not 

 connected with the diacoele (Fig. 30) but is closed by the thick- 

 ening of the endymal cells just caudad of the supracommissure. 



