No. I.] THE VERTEBRATE EAR. 183 



carefully : " The earliest stage at which the auditory involution 

 can be readily recognized occurs soon after the closure of the 

 neural groove in the fore part of the embryo." It may be 

 recognized as a perfectly distinct though shallow invagination, 

 lined by a columnar epithelium belonging exclusively to the ner- 

 vous layer of the ectoderm. At first the edge of the saucer is 

 nearly circular, but soon becomes irregular and narrowed as the 

 saucer is converted into the flask condition. Van Bambeke has 

 figured and described these stages for Pelobates fjisciis, and 

 these two amphibious forms agree very closely in the details of 

 the process. In sections Villy claims to have traced the audi- 

 tory nerve into continuity with the somewhat thickened plate of 

 cells before it has commenced the formation of the saucer. The 

 flask stage is not so perfect as in the Elasmobranchs, and does 

 not long persist, as the mouth of the involution is soon closed 

 and the auditory vesicle loses its connection with the nervous 

 layer of the ectoderm. Villy was not able to describe from his 

 study of this process the details of the closing of the mouth of 

 the flask with exactness, and there remains still some doubt as 

 to the completeness of the closure. This dorsal region or neck 

 of the flask is much smaller than the ventral portion and 

 becomes ultimately the recessus labyrinthi. Villy says {loc. cit. 

 p. 525) : "The outer wall of the vesicle, being the latest formed, 

 consists of cells at first indistinct in outline, but soon they 

 assume, more or less completely, the columnar character of the 

 opposite wall, though the two parts never agree exactly, as the 

 older contains more pigment than the younger. It is from 

 the old pigmented parts that the sensory tracts of the adult ear 

 are formed." The cells of the older part are also more elon- 

 gated than the others. The vesicle now becomes surrounded 

 by the mesoblast in the usual manner. By the time the larvae 

 have grown to the length of 12-14 nim., the canals. begin to 

 appear, but already a separation has been effected between the 

 two parts of the sensory patch, and thus the two chambers of 

 the ear are marked off. They are the utriculus and sacculus, 

 and this part of the ear is now in the Cyclostome stage of 

 development. The appearance of the utriculo-saccular partition 

 belongs to the stage of development when the animal has 

 reached a length of about 11 mm. This partition first appears 

 as a ridge, which by broadening finally divides the auditory 



