298 ROBERT J. TERRY 



The hjnpochordal arch of the atlas is composed of young carti- 

 lage, least developed toward the midplane and continuous, right 

 and left, with the lateral masses. 



In embryos of 17 to 20 mm. (fig. 8) the basal plate is chondri- 

 fied from side to side. Boundaries of the jugular foramina are 

 established by the presence of the basicochlear commissures 

 and the more complete union of the lateral occipital arches and 

 the otic capsules. In relation to the hypoglossal foramen, the 

 jugular tubercle appears anterior, the paracondyloid process 

 lateral; the one within the cranium, the other outside. The basal 

 foramina are all relatively large. A long, wide basi-cochlear 

 fissure is limited anteriorly by the basi-cochlear commissure 

 which separates it from the carotid foramen. The tectum pos- 

 terius is represented in cartilage by a spur springing on either 

 side from the parietal plate. The hypochordal ai-ch of the 

 atlas is complete from side to side. The dens epistrophei is 

 further chondrified cranially. 



Otic region. Embryo of 23.1 mm. 



The otic capsule, externally, presents two well defined sub- 

 divisions, one serving as the framework of the semicircular 

 canals, the other enclosing the cochlear canal (figs. 1, 2, 3). 

 The vestibule, on account of its intimate relation both with the 

 pars canalicularis and the pars cochlearis, will be included in 

 the description of these subdivisions. 



Regarding the position of the otic capsule at the stage repre- 

 sented by the model, a departure from the manmialian type is to 

 be noted. The long axis of the whole auditory capsule of mam- 

 mals is, as Gaupp has indicated, du-ected from behind and 

 laterally, forward and medially; just the opposite of what 

 occurs in lower vertebrates. The long axis of the otic capsule 

 of the cat embryo of 23.1 mm. is inchned somewhat trans- 

 versely (figs. 1, 2). It is true that the broader canahcular part 

 reaches caudally a httle way beyond the cochlear division, but 

 in the cephalic direction both attain almost to the same level. 



Regarding the relation of the otic capsule to the walls of the 

 cranium, the pai's cochlearis lies wholly in the floor and the pars 



