1 68 AYERS. [Vol. VI. 



human embryo in a manner entirely in harmony with Rathke's 

 account. 



Villy (1890, 281) described the development of the frog ear, 

 and devoted especial attention to the early stages of the for- 

 mation of the canals and the division of the sense organs. 



Krause (1890, 169^) criticises Riidinger's observations, and 

 proves that Riidinger had mistaken the ductus endolymphaticus 

 in his sections for the external canal. Krause has thoroughly 

 worked out the history of the ear canals, and he is the first to 

 recognize that, in the Mammalia, the ampullae are marked out 

 before the canals are formed. They are formed, as I can state, 

 as sense-organ pockets. He, however, does not recognize the 

 significance of his discovery in its bearing upon the phylogeny 

 of the ear sense organs. 



During this time the study of the adult anatomy of the in- 

 ternal ear was making great progress. Our knowledge in this 

 department was very incomplete until about the year 1865, 

 which, though only an approximate date, may serve to mark the 

 beginning of the more recent movement in the study of the 

 vertebrate ear which has culminated in that magnificent out- 

 come of the united labors of many of the keenest scientific 

 minds of the last quarter of the century, brought together, 

 critically revised and combined with a very large number of 

 original observations, remarkable for their accuracy and com- 

 pleteness of detail, — entitled Das GeJwrorgan der Wit^belthiere 

 by Gustav Retzius. 



Development of the Ear in Cyclostomes. 



The developing auditory vesicle of Petroniyzon flnviatilis pre- 

 sents many interesting characters which have been only recently 

 made known to us by Kupffer in his beautiful study of this 

 form. The capsule invaginates from the ectoderm as one of a 

 series of invaginations occurring along the side of the head in 

 connection with the formation of the cranial ganglia of the 

 lateral group. The Trigeminus invaginations are most con- 

 spicuous, and give one the impression that we have to deal 

 with the formation of similar structures. Kupffer calls atten- 

 tion to this resemblance, but does not conclude that these are 

 strictly homologous. He says {loc. cit. p. 522): "Das Verhal- 

 ten der Epidermis zu dem massiven Ganglion \i.e. the ganglion 



