78 IOWA ACADEMY OF SCIENCES. 



auditory stimulation. But the most significant fact, it 

 seems to me, is that the pars basilaris is either absent or 

 very imperfectly developed. This structure which finally 

 evolves into the cochlea of the higher Vertebrates, is dis- 

 tinctively the organ of hearing, according to recent physi- 

 ological interpretation. 



We then, in all probability see in this transition class of 

 Vertebrates, the Amphibia, the origin of an organ of hear- 

 ing from an organ of equilibration, which latter function 

 is always retained. To be sure it may be suggested that the 

 lack of a pars basilaris in Proteus, Necturus, etc., may be 

 due to degeneration. Admitting this possibility, yet as I 

 have elsewhere shown, in the development of the ear of the 

 Salamander, Amhl if stoma, the pars basilaris is the last of 

 the parts of the labyrinth to be differentiated, and then 

 only near the close of the larval period. That is, the order 

 in which this structure appears in the embryo, undoubt- 

 edly corresponds to the way in which it originated in 

 the Amphibians as they were evolving from the ancestral 

 aquatic condition into the semi-terrestrial state of existing 

 species. 



A COMBINATION OF CHROMIC ACID, ACETIC ACID 



AND FORMALIN AS A FIXATIVE FOR 



ANIMAL TISSUES. 



H. W. NORRIS. 



That many of the fixing reagents in common use in the 

 preparation of tissues for histological study are poorly 

 adapted to the fixation of the ova and embryos of Amphib- 

 ians, is evident to any one who gives more than casual 

 attention to the matter. The large amount of yolk pres- 

 ent in the cells of the Amphibian embryo makes completely 

 uniform fixation with some reagents impossible, and then, 

 too, the disintegrating effect of many fixing fluids upon 

 the yoke granules makes the result very unsatisfactory. 

 A fluid that gives good fixation is likely to interfere with 



