PRIMORDIAL CRANIUM OF THE CAT 365 



higher plane, looked inward, was traversed by the perilymphatic 

 duct and was recognized as the aquaeductus cochleae. Follow- 

 ing his observation, Fischer states that Hertwig's enlarged 

 model of the otic region of man shows obscurely the process in 

 question. I find in this model a free corner of the cochlear wall 

 projecting forward from the posterior margin of the large fora- 

 men so as to divide the opening into a large lateral and a very 

 small medial part. In the rabbit, Voit found the conditions 

 exactly as Fischer saw them in Semnopithecus and named the 

 dividing process the processus intraperilymphaticus. Macklin 

 ('14) described the intraperilymphatic process in the human 

 embryo as a ''short conical projection directed forward from the 

 inferior utriculoampullary prominence." In the cat, a process 

 of the caudal wall of the cochlear capsule, adjacent to the 

 basivestibular commissure, projects laterally, and tends to sepa- 

 rate the perilymphatic foramen into two parts. We find, there- 

 fore, that the perilymphatic process in the ape and the rabbit 

 is directed backward, and in man forward, while in the cat the 

 process which separates the perilymphatic foramen into intra- 

 and extracranial openings is directed laterally. If these be 

 comparable processes they probably indicate, merely, differences 

 in the place where chondrification begins in the septum dividing 

 the perilymphatic foramen. 



Cavum vestibulare. It is apparent from the foregoing de- 

 scription of the vestibular cavity that the conditions present in 

 the embryo of 23.1 mm. are far from what obtains in the adult 

 mammaUan temporal bone. Only on the most general lines 

 are the form and relations seen in .-the bony walled vestibular 

 spaces of adult cat referable to the conditions of the cartilaginous 

 jotic capsule. Such characteristics of adult structure as the sharp 

 deUmitation of special recesses for the utricle and saccule, ap- 

 parently are not even indicated in embryos considerably further 

 developed than those of the stage modeled. Between these 

 early stages and adult conditions many processes of formation 

 are involved, about which almost nothing is known. 



If the vestibular cavities of the cat embryo of the stages de- 

 scribed present but few indications of their adult form, they 



