629 



such forms as Lacertaagilis to that in Crocodiles, Birds and 

 Mammals, the transition is easily understood. 



Dr. Alexander accepts the homology of the lagena of fishes with 

 that of Amphibia, and also the homology of the lagena throughout 

 the groups Reptilia, Aves, and Mammalia, but he believes that a lapse 

 occurs between the Amphibia and the Reptilia. His investigations 

 in these groups were apparently confined to Ran a fusca and Tro- 

 pidonotus natrix, and his conclusions are chiefly based on his 

 observations on a mammal, combined with a study of the descriptions 

 of the structure in lower forms, as found in the writings of Hasse 

 and Retzius. 



In investigating the development and structure of the pars inferior 

 of the labyrinth of the guinea-pig (Cavia cobaya), he found that during 

 ontogeny there is developed a sensory area within the ductus reuniens. 

 It is at first connected with the macula sacculi, but afterwards se- 

 parates from it, and persists for a time as an independent sensory area, 

 which he proposes to call the macula ductus reunientis. It never 

 possesses a nerve supply, however, and degenerates during the embryonic 

 period. An examination of his excellent figures leaves no doubt as 

 to the existence of this transitory macula acustica, but his interpre- 

 tation of its significance seems to me wholly unjustifiable. 



He says that it can certainly be no new structure, and that it is 

 probably to be found in a well developed condition in other Verte- 

 brates: "Wollen wir daher für die Maculaanlage des Ductus reuniens 

 und durch diese für den Ductus selbst im Bereiche des Wirbeltier- 

 labyrinths ein Homologon aufstellen, so ist nach dem oben gesagten 

 nur zwischen Macula lagenae und Macula neglecta, bezw. zwischen 

 Lagena und Pars neglecta zu entscheiden." He then gives a brief 

 summary of the phylogeny of the lagena and pars basilaris, together 

 with a series of diagrams representing his conception of the course 

 of evolution of the cochlea. W T ith reference to his Fig. 2 (in the text) 



the latter group "macula basilaris" is more appropriate, and the term 

 macula lagenae is applicable to the sensory area of the lagena in all cases. 

 The evolution of the papilla basilaris from the Reptilia onwards 

 supports the view that it is the vibrations of the basilar membrane and 

 its overlying papilla, aüd not the vibrations in the endolymph, that are 

 of the greatest importance in the conversion of stimuli. Otherwise there 

 seems no adequate reason for the conspicuous progressive evolution of 

 the papilla basilaris, after it has taken up its new position in the 

 Reptilia. In saying this I do not lose sight of the fact that the 

 endolymph of the scala media must also have its part to play. 



