SKULL OF A HUMAN FETUS OF 40 MM. 357 



The condition of the terminal portion of the ductus endolym- 

 phaticus is of interest. This does not end in a sac, but becomes 

 a long, narrow fusiform dilation shortly after emerging from the 

 foramen, and gradually decreases in size, to be prolonged, at its 

 dorsal extremity, into a fine, lumenless filament or cord of cells. 

 After leaving the foramen endolymphaticum it passe's medial 

 to the transverse sinus in the sub-dural space, outward and back- 

 ward, and ends in the loose sub-dural connective tissue just 

 medial to the occipitoparietal groove, about 1.8 cm. dorsal to 

 the capsuloparietal fissure. It is not intimately associated 

 with the cartilage of the ear capsule after its exit therefrom, 

 and hence cannot retard the development of this locally to 

 bring about a thinness of the wall, which is found in my prepara- 

 tion just dorsal to the endolymphatic foramen, in the area 

 corresponding to that in which the small foramen which Voit 

 describes in the developing otic capsule of the rabbit appears. 

 This thin region of the wall (which is unperforated) is caused by 

 encroachment upon it from within of the cavities surrounding 

 the dorsal extremities of the anterior and posterior semicircular 

 canals, and not by pressure of the saccus endolymphaticus from 

 without, as Voit assumes in the skull of lepus. 



The upper part of the medial surface is marked by the crescentic 

 inner aspect of the anterior semicircular prominence (fig. T), 

 which is more distinct here than on the lateral surface, and 

 sweeps backwards, from the superior utriculoampullary promi- 

 nence to the dorsal end of the prominence of the crus commune. 

 It corresponds to the arcuate eminence of the adult bone. Below 

 this prominence is to be seen a distinct fossa, the fossa sub- 

 arcuata anterior (Voit), delimited caudally by the prominence 

 of the crus commune. This fossa invades the substance of the 

 massa angularis, and upon examining the slides microscopically 

 it is found that it is filled with a mass of loose connective tissue, 

 covered by the dura. 



The medial surface below the prominence of the crus commune 

 looks downward, backward and inward in its upper portion, and 

 almost directly downward in the lower. The latter is thin, com- 

 posed of more darkly staining cartilage with thickset cells and 

 little ground substance, and forms the roof of the supraalar 



