326 Robert J. Terry. 



to be seen in both the end-vesicle and stalk, the clear central 

 portion of the latter extending quite to the base and to the margin 

 of the ventricle. In later stages a cavity forms, taking the place of 

 the region of lightly-staining protoplasm, and the peripheral nu- 

 cleated layer then becomes a surrounding wall. The thickness of 

 the peripheral layer varies somewhat in different parts of the ves- 

 icle, but there is no marked contrast in this respect between the 

 anterior and posterior regions of the layer. In transverse (fig. 

 3, ES) and in frontal sections, the epiphysis is seen to have a 

 median position. The point of union of the stalk with the roof 

 of the third ventricle is also midway between the intermediate 

 tubercles; that is to say, there is no approximation of one or the 

 other of the tubercles toward the middle line. Regarding the pres- 

 ence of a second epiphyseal organ, the following observation was 

 made. In the series of transverse sections no. 121, a small rounded 

 body was found to the left of the median line anterior to the 

 epiphysis and surmounting the superior commissure (fig. 4, A). 

 In section no. 101 (fig. 5, A) continuity was traced between 

 this body and the roof of the third ventricle anterior to the com- 

 missure. Structurally it consists of a peripheral nucleated stratum 

 surrounding a central clear area of protoplasm. 



The depression on the dorsal aspect of the brain between the 

 mesencephalon and telencephalon is filled with a loose mesenchy- 

 mal network. This tissue surrounds the epiphysis and connects 

 the end-vesicle with the membranous roof of the cranium by a 

 dense broad band. Hill states that the end of the epiphysis in 

 Salmo of 13 mm. projects into a mass of undifferentiated tissue 

 lying between it and the epidermis. Blood vessels are present at 

 the sides and at the back of the epiphyseal stalk. One of them is 

 constant and traverses the mesenchyma in the median plane. 

 The pigmentation of the tissues about the epiphysis described by 

 Cattie ('82) appears in the later stages of development of Opsanus. 

 Differentiation of the epidermis over the pineal region does not 

 occur except in connection with the formation of the pit organs 

 of the lateral line system. 



Superior Commissure. Fig. 2, taken from a median sagittal 

 section of the pineal region of an Opsanus embryo of 11 mm., 



