24 NORR/S. [Vol. VII. 



laboratory of the University of Nebraska during the year 

 1890-91. 



Early Stages in Development of the Ear. — Shortly after the 

 differentiation of the neural ridge the sensory layer of the 

 ectoderm in the region of the hind brain between the Anlagen 

 of the VII.-VIII. and IX. nerves begins to thicken, forming on 

 either side a patch of columnar nucleated cells destined to give 

 rise to the auditory vesicle. Sections through this region at 

 this period (Fig. i) show the outgrowth of the neural ridge, 

 later forming the VII.-VIII. nerve, in close proximity to the 

 above-mentioned thickened patch of ectoderm. Invagination of 

 this ectoderm soon occurs (Figs. 2 and 3), indicated on the exte- 

 rior by a slight depression of the indifferent ectoderm, though 

 the latter takes no share in the formation of the ear vesicle. 

 The nuclei of the cells of the invaginating area lie at the inner 

 ends of the long columnar cells, and the latter are for the most 

 part arranged in one layer. The invagination becomes vesicu- 

 lar by the edges of the pit, formed in the infolding process, 

 approaching each other and finally coalescing. The closing of 

 the mouth of the pit begins at the ventral side and progresses 

 dorsally, so that the dorsal part of the ear vesicle is the last 

 to close off from the space beneath the indifferent ectoderm. 

 This dorsal portion is the recessus, and it is thus homologous 

 with the recessus in those forms in which the vesicle never 

 loses its connection with the exterior. Villy ^ states that in the 

 frog the recessus is not the last part of the vesicle to retain its 

 connection with the external skin, but results merely from the 

 mode of involution. In Amblystoma at the time the vesicle 

 becomes a closed sac its small cavity is elongated dorso-ven- 

 trally and curved with the concavity directed anteriorly. A 

 curved line would then mark the coalesced lips of the involuted 

 ectoderm. Later, the vesicle becomes pyriform with the 

 smaller end directed dorsally (Fig. 4). This dorsal portion, the 

 Anlage of the recessus, is thin walled and consists of one layer 

 of cells. By rapid growth of the lateral wall of the vesicle the 

 recessus is pushed toward the brain, and as it becomes more 

 and more marked off from the rest of the vesicle opens into 

 the latter at the dorso-mesal border. This is the condition 



^ Development of the Ear and Accessory Organs in the Frog, Quart. Jour. Mic. 

 Sci., No. CXX., 1890. 



