Sorensen, Study of Epiphysis and Roof of Diencephalon. 3 3 



Beard studied one specimen of Myxine in which the struc- 

 ture of the organ in question could be well made out. The eye 

 is a large flattened organ lying within the skull and connected 

 to the thalamencephalon by a short, thick, solid stalk. The ves- 

 icle contained no pigment. The epiphysis in Myxine differs 

 from that in Petromyzon in that it is not divided into two vesi- 

 cles. Both anterior and posterior walls have the same struc- 

 ture. 



The two forms Myxine and Bdellostoma are very closely 

 allied, and in other points of great importance such as the 

 structure of the teeth, they closely resemble each other and 

 form a contrast to the Petromyzontidae, which while in some 

 respects less degenerate than the former are in others less prim- 

 itive. In concluding his article Beard calls attention to the fact 

 that in Cyclostomata the parietal eye has all the characteristics 

 of a degenerate organ, one especially in a very high degree, 

 viz., its variability in different individuals. 



McKay (55-88) studied the development and structure of 

 the pineal eye in Hinulia and Grammatophore and described 

 the external appearance of the eye in embryos as follows : In 

 the embryos of Hinulia which were advanced in development, 

 the pineal eye could be seen standing out as a projection, at a 

 point where the anterior joined the middle cerebral vesicle. 

 The projection viewed as a solid object appeared to be com- 

 posed of two lobes, an anterior larger, and a posterior smaller 

 one. This appearance was explained when a longitudinal sec- 

 tion of the head had been made ; the anterior lobe was seen to 

 be the pineal eye proper, while the posterior lobe was the curved 

 end of the epiphysis. In the advanced stages of Hinulia, the 

 eye appeared as a black spot in the median line of the head 

 posterior to the paired eyes. 



Three stages in the development of Grammatophora muri- 

 cata are described. In the first stage the epiphysis cerebri, 

 composed of a single layer of columnar cells with well-marked 

 nuclei, arises as an outgrowth of the thalamencephalon. In the 

 second stage the anterior wall grows forward and forms a sec- 

 ond evagination in the wall of the primary one. Thus we have 



