314 GEORGE L. STREETER 



the feiiestni cochleae (rotuiHlmii), just as the cistern forms 

 opposite the stapes and the fenestra vestibuU. The scala 

 tympani always begins at the same place and extends downward 

 along the cochlear duct, at first a little in advance of the scala 

 vestibuH, but subsequently the latter catches up with it and 

 the two reach the tip of the duct at about the same time. 



It is well known that the proximal portions of the cochlear duct 

 mature sooner than the distal portions. One might expect that 

 the accompanying periotic spaces would correspond in their 

 development to the maturity of the duct and therefore the proxi- 

 mal parts of the scalae would differentiate first. In other words 

 the maturation of the cochlea proceeds as a wave from the proxi- 

 mal end to its tip involving all of its constituent structures as 

 it passes along, including mesenchymal parts as well as epithelial. 

 This conception might explain the direction of development 

 of the scalae but it can hardly be applied to the cistern, the 

 vestibular representative of the scala vestibuli. One cannot say 

 that those portions of the membranous labyrinth lying opposite 

 the focus of development of the cistern, that is, the lateral 

 walls of the saccule and utricle mature in advance of the rest 

 of the labyrinth. There is no indication that a w^ave of differ- 

 entiation passes through the epithelial elements of the laby- 

 rinth in the same direction and synchronously with the extension 

 of the cistern as it advances from its primary focus up on the 

 roof of the utricle and over on its median surface. In the case 

 of the cistern it seems much more likely that the point at which 

 it first appears is determined by the position of the stapes, which 

 is doubtless an expression of the physical relation that subse- 

 quently exists between the two. By analogy this would yield 

 additional significance to the relation existing between the 

 fenestra cochleae and the point of beginning development of 

 the scala tympani. 



The form and relations of the scalae in fetuses between twelve 

 and thirteen weeks old are shown in figures 6 and 7. These 

 figures show median and lateral views of a wax-plate reconstruc- 

 tion of the membranous labyrinth and the surrounding periotic- 

 ular spaces in a human fetus 85 mm. CR length (Carnegie Col- 



