354 ROBERT J. TERRY 



That the condition in Talpa is primitive, and not secondarily- 

 acquired, is supported by Gaupp's ('08) observation of a simi- 

 lar atlanto-occipital articulation in Echidna. Whereas, in cat, 

 the occipito-atlantal articulation is apparently dicondylic pri- 

 marily, the joint surfaces are located to a considerable extent 

 upon the basal plate, as well as on the lateral arch, and meet 

 corresponding surfaces of the ventral arch of the atlas and its 

 lateral mass, including the atlantal process. The participation 

 of the basal plate in the articular surface is evidence of greater 

 proximity of the two condylar surfaces toward the median plane 

 than in those mammals (Lepus) where the surfaces are restricted 

 to the region lateral to the foramen magnum. This embryonic 

 state of the condyles is, therefore, apparently intermediate be- 

 tween typical dicondylism and the condition observed by Fischer 

 in Talpa. In the later development of cat embryos the condy- 

 lar surface grows further dorsad by the side of the foramen 

 magnum. 



If we accept the comparison of occipital and atlas, we must 

 throw aside the idea of that form of articulation between them, 

 such as exists between the centra of typical vertebrae. 



The basal plate in the cat is a derivative of the parachordal 

 cartilages and hypochordal arches and therefore the develop- 

 ment of an articular surface upon its caudal margin must in- 

 volve either one or both of its constituent elements. The point 

 to be emphasized now is the fact that a part of the occipital 

 element, which apparently is as distinct from the centrum as the 

 lateral mass is distinct from the body of the atlas, enters into the 

 constitution of the condyle; that is to say, the condyle belongs 

 to an arch structure. 



Further study of that region dorsad of the condyle which 

 is characterized by a notch traversed by veins is necessary 

 before any conclusion can be drawn regarding its significance; it 

 seems not improbable that it may have to do with the foramen 

 condyloideum. 



Plane of the foramen magnum. Although a marked flexure 

 exists between the head and trunk of cat embryos, this does not 

 explain the basal position of the plane of the foramen magnum 



