No. I.] DEVELOPMENT OF THE EAR OF AMBLYSTOMA. 25 



when the larva has reached a length of about 9 mm. (Fig. 5), 

 At this time the main cavity of the vesicle is circular in cross- 

 section, somewhat elongate longitudinally and tapering anteri- 

 orly. The auditory nerve at this time is not distinct from the 

 main VII.-VIII. trunk. Its ganglion is situated at the ven- 

 tro-mesal border the vesicle. The lateral wall of the vesicle at 

 the time the auditory involution becomes vesicular is closely 

 connected with the sensory layer of the ectoderm. Later, 

 amoeboid mesodermal tissue pushes in between the vesicle and 

 the ectoderm, and the ear gradually recedes farther and farther 

 from the exterior as development goes on. At the time the 

 larva has reached a length of 9 mm. (Fig. 5) a portion of the 

 ventral, lateral, and posterior walls of the vesicle is modified 

 into a thickened patch of cells destined to form the sensory 

 epithelium of the ear. At this period the entire vesicle con- 

 sists of columnar nucleated cells arranged for the most part, 

 except in the above-mentioned thickened patch, in one layer. 



The Semicircular Canals, the Utriciilus, and the Sacciiliis. — 

 In larvae of 9 mm. in length (Fig. 5) no indication of semicir- 

 cular canals can be discerned. Shortly after this (Figs. 6 and 

 7) there appear four protuberances or outpushings of the walls 

 of the vesicle, three of them being the Anlagen of the semicir- 

 cular canals, while the fourth marks the beginning of the 

 lagena. At this time there can be distinguished utricular and 

 saccular portions (Fig. 16), not, as Villy^ described in the frog, 

 by an oblique partition (for this is not yet formed) in the poste- 

 rior part of the vesicle, but merely in general outline. As yet 

 no division in the form of folds or constrictions can be seen 

 between utriculus and sacculus. This early indication of a 

 division is, as Ayers ^ has stated to be typical of all Vertebrates, 

 of an anterior utriculus and a posterior sacculus. From sec- 

 tions alone it is not easy to see this utriculo-saccular division, 

 but it becomes very evident from reconstructions. The canals 

 are formed, with the exception as to time and order of appear- 

 ance, in the manner typical of Vertebrates. Incomplete parti- 

 tions formed by the coalescing of folds of the walls of the vesi- 

 cle shut off the cavities of the protuberances from the main 



1 Loc. cit. 



^ The Ear of Man : Its Past, Its Present, and Its Fttttcre, Biological Lectures, 

 Marine Biological Laboratory, Vol. L Lect. IX, Boston, 1891. 



