138 AETHUR DENDY. 



vesicles— is seen the posterior epiphysial outgrowth. It is 

 a small spherical body, having its lateral walls thickened so 

 that the cavity within it is laterally compressed. This cavity 

 is narrowest at the middle, on account of the greatest thickness 

 of the lateral wall of the vesicle falling at the middle of its 

 antero-posterior axis ; consequently, in a dorsal view the 

 cavity has the form of a dumb-bell. Just in front of this 

 vesicle, and a little to the left of it, is a second similar 

 outgrowth. This anterior evagination is smaller than the 

 posterior one, and appears to be solid. It lies close against 

 the wall of the posterior vesicle, and is partly hidden by it.'^ 



At earlier stages Hill found that each vesicle arises as a 

 separate outgrowth from the roof of the brain, and each contains 

 a cavity which opens separately into the cavity of the brain. 

 "The anterior vesicle shows an increase in size for about 

 twenty days, and after that a decrease, while the posterior 

 vesicle shows from the beginning a gradual increase." The 

 author further compares these two " epiphysial outgrowths " 

 with those described by Leydig ^ in Lacertilia, and points out 

 " that while the early stages of these two epiphysial outgrowths 

 of Coregonus agree in many details with the corresponding 

 early stages of the two outgrowths in Lacertilia, as described 

 by Leydig, yet the ultimate fate of these two outgrowths in 

 the two forms is widely different. In Coregonus the anterior 

 outgrowth, which is the smaller, gradually disappears, while 

 in Lacertilia, according to Leydig, it develops into the adult 

 parietal organ. '^ He further thinks it probable that the two 

 outgrowths will be shown to be homologous with the primary 

 and secondary parietal vesicles described in adult Petromyzon 

 by Ahlborn (1), and in Lacertilia by Ritter (29). 



In a later paper (17), published in 1894, Hill continues his 

 excellent researches on this subject, and arrives at closely 

 similar results in the case of various fishes. He shows that 

 there are two epiphysial outgrowths from the roof of the 



' " Das Parietalorgan der Ainphibien und Reptilien," ' Auatomische-histo- 

 logiscbe Untersuchung,' Senckenberg. Naturf. Ges., Band xvi, p. 441. I 

 regret tbat I have been unable personally to consult this important memoir. 



