No. I.] THE VERTEBRATE EAR. 169 



of the Trigeminus nerve] ist in Zeitptinkte der Abscliniirimg 

 beider Gebilde gans ilbereinsiimmend." Tiiese invaginations are 

 connected by a ridge of thickened epithelial cells. I regard this 

 as proof of the strict homology of the ear sense organs with 

 the sense organs belonging to the Trigeminus and Vagus groups 

 of nerves. Soon after the invagination, the internal ear of 

 Petromyzon presents a round appearance, and by the sixth day 

 has assumed a somewhat hemispherical shape. With a dorso- 

 median portion of the auditory flask one of the branches of the 

 dorsal root of the auditory nerve has come in contact. Later 

 on there appears a fissure in the part of the wall of the auditory 

 vesicle, and the auditory nerve makes its way into its future 

 internal territory of distribution. 



In Shipley's (1887, 261) paper on the development of Petromy- 

 zon, he describes the growth of two thickenings of the epithelial 

 wall which arise from the anterior and posterior inner angles 

 (if one may speak of these portions of the curved walls as 

 angles) of the auditory vesicle. The formation of the canals is 

 a later phenomenon, and they are first marked off, as Scott has 

 found, by the outgrowth of two folds from the outer and inner 

 walls of the vesicle which grow toward each other. The inner 

 process is situated higher up and just external to the recesstis 

 labyrinihi. In the next stage the two processes are in contact, 

 and then coalesce, dividing the vesicle by means of an oblique 

 partition into two chambers. In the middle of the dorsal wall 

 the partition is incomplete, and here the two chambers com- 

 municate. 



Kupffer found in the larvae of Petromyzon Planeri (Ammo- 

 coetes) that in its earliest stage the auditory involution is rep- 

 resented by " eine etwas unregelmassige Einsenkung der 

 Epidermis, deren Zellen hier etwas verlangert sind und durch 

 den Vorgang, der sich vollzieht, eine Aenderung ihrer Gestalt 

 erfahren. Dadurch wird der eingesenkte Abschnitt etwas dicker, 

 als die nachste Umgebung. 



"Ventralwarts hiervon erscheint eine neue Epidermisleiste, 

 die sich durch Verlangerung der Zellen und mitotische Figuren 

 bemerklich macht. 



" Die Nervenanlage besteht aus einer Reihe dicht an einander 

 gelagerten Zellen, die aus der Decke des Hinterhirns hervor- 

 geht und ein freies Ende hat, denn die letzte konisch gestaltete 



