SKULL OF A HUMAN FETUS OF 40 MM. 367 



cast includes not only the membranous labyrinth but the space 

 surrounding it, together with the entrances of the various foram- 

 ina. In the illustration the laminopyramidal commissure ap- 

 pears as a foramen behind the coiled portion of the cochlear 

 tract. In general form the cast resembles the later osseous 

 labyrinth. 



The cartilago supracochlearis (figs. 1, 3, 13) may now be 

 considered. This is a small, rounded mass of cartilage, situated 

 upon the cranial pole of the pars cochlearis, and, in the model, 

 is about 8 mm. wide, and almost as 'long dorso-ventrally. It 

 is quite free from cartilaginous union with the underlying cochlea, 

 but the two are more closely approximated posteriorly than 

 anteriorly, where the intervening connective tissue is thicker. 

 The cartilage is immediately beneath the anterior part of the semi- 

 lunar ganglion (fig. 13), and the material of which it is formed is 

 mature cartilage of apparently the same age as that in the ad- 

 joining pars cochlearis. 



It is difficult to say what may be the significance of this carti- 

 lage. Certainly it cannot be any one of the Restknorpeln which 

 Voit describes in his Stage II (43 mm.) of the rabbit, since only 

 one of these, Restknorpel b, corresponds at all in position with 

 this cartilage, but it is distinctly above the semilunar ganglion 

 while the siipracochlear cartilage is below it. I am inclined to 

 regard it as a rudiment of the commissura (or trabecula) ali- 

 cochlearis, which Jacoby describes in his 30 nrni. human embryo 

 as a cartilaginous bridge extending between the anterior part 

 of the pars cochlearis of the otic capsule and the ala temporalis. 

 There is no evidence in my model of such a commissure, though 

 the surfaces of the processus alaris of the temporal wing and the 

 ventral surface of the pars cochlearis are very close together, 

 and in the later stage modelled by Hertwig (80 mm.) there is no 

 evidence of either commissure or rudimentary cartilage in this 

 location, indicating that the cartilage in my embryo is probably 

 undergoing retrogression. Voit describes and figures such a 

 commissure in the skull of the rabbit, which encloses the carotid 

 foramen laterally. He states it is a direct forward continuation 

 of the planum supracochleare of the pars cochlearis. 



