298 ARTHUR WILLEY. 



the same series of cells with the latter. This exact primary 

 relation of the otocyst to the eye-cells was not observed by 

 Kowalevsky, and it is interesting as showing that two such 

 different organs as an eye and an ear can arise in the same 

 way — namely, by a deposition of pigment — from one and the 

 same sense- tract. 



Next begins the migration of the otocyst^ which was 

 discovered by Kowalevsky. This migration is not an active 

 one on the part of the otocyst, but goes hand in hand with a 

 change in the histological character of the wall of the cerebral 

 vesicle, and is therefore, as in so many other cases of change 

 in the position of organs, a result of the relative differential 

 growth of parts. This histological change in its turn is 

 correlated with the expansion of the original slight anterior 

 dilatation of the nerve-tube into a spacious vesicle. Successive 

 stages in this expansion of the cerebral vesicle are shown in 

 figs. 1 and 3 — 5. In fig. 4 the otocyst is seen to have separated 

 itself from its previous contact with the cells of the optic 

 region, and now lies at a lower level in the right dorso-lateral 

 region of the cerebral vesicle. In fig. 5 the interval between 

 the otocyst and the eye has further increased itself by the re- 

 duction of the wall of the cerebral vesicle in this region to a 

 thin and apparently structureless cuticle, the reduction in bulk 

 being accompanied by an increase in extent. 



In this way, therefore, by a local thinning out or cuticulari- 

 sation of the wall of the cerebral vesicle the otocyst is shifted 

 from its primary dorsal position to its secondary position on 

 the floor of the cerebral vesicle. The migration of the otocyst, 

 therefore, occurs concurrently with the reduction to a thin 

 membrane of part of the wall of the cerebral vesicle. The 

 mode in which the otocyst becomes transported from the 

 dorsal to the ventral wall of the cerebral vesicle was not made 

 quite clear by Kowalevsky, who says (loc. cit., p. 117), ''^Die 

 vordere pigmentirte Zelle, welche wir vermuthlich als einen 

 Gehorapparat gedeutet haben, schiebt sich von der rechten 

 Wand der Blase nach unten, so dass sie auf den Boden der 

 Blase kommt.^' From this description it might very natur- 



