50 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE. 



The earliest histological evidence of the formation of the periotic spaces occurs 

 near the stapes, in the reticulum that bridges the interval between the sacculus 

 and the fenestra vestibuli. In embryos between 30 mm. and 40 mm. long, it can 

 be seen that the meshes in this region are becoming irregular and larger, due to the 

 disappearance of some of the trabeculse and a consequent coalescence of the inter- 

 trabecular spaces. The widening of the mesh at this point constitutes the primor- 

 dium of the vestibular cistern. It makes its appearance before there is any trace 

 of the scalae, but it is not until the fetus reaches a length of about 50 mm. that the 

 cistern becomes definitely outlined and clearly differentiated from the adjoining 

 reticulum. 



Following the appearance of the cistern, the scala tympani is the next space to 

 become established. It can be recognized as a moderate widening of the meshes 

 of the reticulum in the region of the fenestra cochlea? in fetuses 43 mm. long, along 

 the basal border of the first turn of the cochlear duct. The scala vestibuli, as can 

 be seen in fetuses 50 mm. long, develops as an extension downward of the cistern 

 along the apical border of the cochlear duct. Starting from these definite foci, 

 these three spaces spread into their destined territory, absorbing as they go the 

 enlarging reticular spaces of the invaded region by a process of space-coalescence, 

 or, in other words, the progressive formation of areas that are free of trabeculae. In 

 fetuses 85 mm. long the two scalae extend downward along the cochlear duct to 

 its last turn, as two separate spaces which do not communicate with each other. 

 When they reach the tip of the duct, which occurs in fetuses about 130 mm. 

 crown-rump length, a free opening is developed between them which represents 

 the helicotrema. After being completely established along the whole length of 

 the cochlear duct, the scala? continue to enlarge by further coalescence of tissue along 

 their peripheral border, in which the trabeculae disappear. 



The periotic spaces are analogous in their development to the pia-arachnoidal 

 spaces; they are not, however, extensions of them that have invaded the cavity of 

 the cartilaginous labyrinth. They begin at points where there can be no connec- 

 tion 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 aquaeductus cochleae, is established quite late. In fetuses 85 mm. crown- 

 rump length it exists as a tubular pouch projecting from the subarachnoid 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 com- 

 munication must be established soon after this. 



Similar projections from the subarachnoid spaces at the internal auditory 

 ineatus extend as perineural clefts along the trunk and branches of the acoustic 

 nerve. No actual communications, however, were seen between these spaces and 

 the two scalae. 



