430 C. W. PRENTISS 



organ, but believes that in adult mammalia its attachment to 

 the cells of the organ of Corti is lost. 



Because of these contrary and diverse conclusions which the 

 literature in regard to the development and structure of the 

 membrana tectoria contains, and because of its importance to 

 the physiology of audition it was determined to make a restudy 

 of its development in pig embryos and of its structure in man. 

 The present paper includes my work on the development only. 



METHODS 



Experimenting with fixing fluids it was found that forinahn 

 and Zenker's fluid preserved the membrana well but failed to 

 bring out sharply its cuticular structure. Osmic acid of 2 per 

 cent and Vom Rath's osmic-picric-acetic mixture was used with 

 success in the later stages as the fixation was good and the brown- 

 ing of the cuticulum by the osmic acid made its structure more 

 clear. 



In the younger stages the whole head of the embryo was fixed. 

 In the stages approaching full term the bony labyrinth was shelled 

 out whole, and, after the stapes had been carefully removed, was 

 immersed in the fixative two to three days. After fixation and 

 hardening the decalcification of the older stages was completed 

 in 80 per cent alcohol plus 5 per cent nitric acid. They were 

 then embedded in celloidin or paraffine and cut in planes parallel 

 and perpendicular to the modiolus of the cochleae. Preparations 

 were thus made from pig fetuses measuring 4, 5.5, 7.5, 8.5, 13, 

 15, 18.5 and 20 cm. and these were compared with sections from 

 full term fetuses. It was found that sections mounted in balsam 

 were not favorable for a study of the membrana because its 

 cuticular framework is of about the same refractive index as this 

 mounting medium. Celloidin sections were therefore mounted 

 in water, and while only temporary preparations could be made 

 in this way, this method was of great value in determining the 

 structure of the membrane when unstained. With sufficiently 

 thin sections an oil immersion objective could be employed. No 

 special staining methods were used, the browning of osmic acid 



