;i2() GEORGE L. STREETER 



a I'kh' ()])enmg is developed between them and this represents 

 the helicotrema. 



Tlio perioticnlar spaces are analogous in their development to 

 tlie pia-araclmoidal spaces; they ai"e not however extensions of 

 tliem that liave invaded the cavity of the cartilagenous laby- 

 ivinth. They begin at points where there can be no connection 

 with the arachnoidal tissue and their direction of growth is 

 quite independent of it. The communication that is found in 

 the adult between the scala tympani and the subarachnoid space 

 in the neighborhood of the fenestra cochleae, the so-called aquae- 

 ductus cochleae, is estabhshed quite late. In fetuses 85 mm. CR 

 length it exists as a tubular pouch projecting from the subarach- 

 noid spaces along the glossopharyngeal nerve toward the scala 

 tympani. In the 130 mm. fetus, the oldest examined, this pouch 

 is longer and nearly reaches the scala. The communication 

 must be established soon after this. 



Similar projections from the subarachnoid spaces at the inter- 

 nal auditory meatus extend as perineural clefts along the trunk 

 and branches of the acoustic nerve. No actual communications 

 were seen between these spaces and the two scalae. 



