106 IOWA ACADEMY OF SCIENCES. 



The earliest stage that I have as yet found in the development of the ear 

 of Necturus is that shown in Fig. 1. The auditory involution has just 

 begun. In Fig. 2 the growth has proceeded so far that the pit is nearly 

 closed. After the complete differentiation of the vesicle the ear is of a 

 pyriform shape with the apex directed toward the dorsal part of the brain 

 (Fig. 5). The apical portion soon becomes distinctly marked off from the 

 rest of the vesicle as the recessus labyrinthi (Fig. 6). In Amblystoma I 

 observed that the dorsal side of the primitive pit was the last to close up, 

 thus giving support to the belief that the recessus of the Amphibian ear is 

 strictly homologous to the recessus of the Elasmobranch ear, in which the 

 primitive connection with the exterior is maintained through life. Just the 

 reverse process is said, by Villy, to occur in the frog. In Necturus I have 

 not satisfactorily decided how the recessus is formed. As the vesicle 

 increases in size the recessus becomes more distinctly marked off, its apex 

 grows dorsally till it lies over and upon the brain. Instead of opening into 

 the dorsal portion of the vesicle its aperture is situated on the median side 

 close to the brain (Figs. 8 and 10). The semi-circular canals are formed in 

 the tyi)ical manner. As in Amblystoma the horizontal canal is the first to 

 make its appearance. Folds of the walls of the vesicle grow in so as to 

 imperfectly divide the ear into a number of parts: sacculus, utriculus, 

 semi-circular canals, etc. The beginnings of the processes that result in the 

 differentiation of the various parts of the ear are shown in Figs. 8 and 10. 



The later stages have not been studied in detail, owing partly to lack of 

 material. But this much may be stated with certainty: The ear of Nec- 

 turus in its morphology and ontogeny does not differ in any important 

 respect from that of Amblystoma. Necturus is regarded as representing a 

 more ancestral type than Amblystoma; hence we should expect to find its 

 organs more genex-alized. But it is usually unsafe to base sweeping com- 

 parisons in relationship on the similarities or dissimilarities of single 

 organs. The sense organs connected with the various parts of the ear cor- 

 respond to those in Amblystoma. But of the existence of the 2^(''''>'S basilaris 

 I can state nothing. Retzius^ denies its existence in Proteus, the near rel- 

 ative of Necturus. 



The orders of recent Amphibia are three. Each order has its peculiar 

 modification of the membranous part of the ear. The ear of the Caecilians 

 seems to be the most primitive of these, from the research of the Sarasin 

 Brothers*. I tind in Necturus no vestiges of the peculiarities of the Caecil- 

 ian ear. 



The material on which this paper is based was obtained from Miss Julia 

 B. Piatt, of Chicago University. 



Explanation ot figures and abbreviations used: 



Au auditory involution, auditory vesicle, ear; cmg auditory ganglion; br 

 brain; ca anterior semi-circular canal; ch chorda; ei indift'erent layer of ecto- 

 derm; ent entoderm, roof of mouth; es sensory layer of ectoderm; mes mes- 

 oderm; nr neural ridge; oiJt eye; o aorta; r recessus; sed dorsal fold of 

 septum of horizontal canal; sev ventral fold of septum of horizontal canal. 



Figs. 1, 3, 5, 6, 8 and 10 are camera lucida drawings of sections. Figs. 

 3, 4, 7, 9 and 11 were drawn under the writer's direction from alcoholic 

 material, by Mr. H. G. Willard. 



SDas Gehororgan der Wirbelthiere, Stockholm, 1881-84. 



lUeber das Gehororgan der Caeciliiden, Anat. Anz., Nos. 25 and 26, 1892. 



