No. 3.] EPIPHYSIS IN COREGONUS ALBUS. 509 



onic stage the anterior one is the larger, and partly covers the 

 smaller posterior one. The anterior outgrowth soon separates 

 from the brain, and develops into the eye-like parietal organ, 

 while the posterior one remains connected with the brain roof, 

 and forms the pineal gland (Zirbelknopf). My work on Core- 

 gonus does not cover adult forms, but, as far as studied, it is 

 evident that while the early stages of these two epiphysial out- 

 growths of Coregonus agree in many details with the corre- 

 sponding early stages of the two outgrowths in Lacertilia, as 

 described by Leydig, yet the ultimate fate of these two out- 

 growths in the two forms is widely different. In Coregonus 

 the anterior outgrowth, which is the smaller, gradually disap- 

 pears, while in Lacertilia, according to Leydig, it develops into 

 the adult parietal organ. 



Selenka ('90), in Lacerta and Anguis, also describes two 

 epiphysial outgrowths. The posterior outgrowth, he says, 

 arises as a median thickening of the roof of the thalamenceph- 

 alon. This he calls the epiphysis, and from its distal end he 

 derives the parietal eye. Shortly after this there is another 

 outgrowth, which he calls paraphysis, which comes from the 

 roof of the prosencephalon. The paraphysis then grows back- 

 ward and the epiphysis grows forward in such a way that the 

 pineal eye comes to lie on the paraphysis as on a pillow. I 

 could find no such outgrowth from the prosencephalon in 

 Coregonus. The two outgrowths were both posterior to the 

 depression, which, according to Rabl-Riickhard ('82), marks the 

 anterior border of the thalamencephalon. 



What relation these two epiphysial outgrowths bear to the 

 primary and secondary parietal vesicles, described in adult 

 Petromyzon by Ahlborn ('83) and in Lacertilia by Ritter ('91), 

 remains yet to be worked out. It is probable that these organs 

 will be shown to be homologous. 



Zoological Laboratory, University of Michigan, 

 September 9, 1891. 



LITERATURE. 



'83. Ahlborn, F. Untersuchungen liber das Gehirn der Petromyzonten. 

 Zeitschr.f. wiss. Z00L, Bd. XXXIX., pp. 191-294, Taf. 13-17. 



