352 ROBERT J. TERRY 



tion of its position the bar of bone forming its cephalic Umit is 

 narrow or broad. In man the first spinal nerve and vertebral 

 artery normally traverse a notch in the superior margin of the 

 neural arch; in some instances, however, they pass through a 

 foramen produced by osseous bridging of the notch. Bolk ('99) 

 has pointed out that the usual conditions are indicative of re- 

 gressive processes in the formation of the human atlas, that in 

 man this bone is reduced in mass as compared with the atlas 

 of those animals in which an atlantal foramen obtains. The 

 bony reduction goes hand in hand with the imperfect develop- 

 ment, and probable loss to some extent, of muscle and nerve in 

 the dorsal part of the neck at the level between epistropheus and 

 occiput. The recognition of the human atlas as an atypical 

 example of the form which generally prevails in mammalia is 

 helpful in approaching problems of structure in the head-neck 

 region. Out of the recently much discussed phenomenon of 

 manifestation of an occipital vertebra or assimilation into the 

 occiput of the atlas (Swjetschnikow '06, Kollmann '07, v. Schu- 

 macher '07, Smith '09, Glaesmer '10), the question has presented 

 itself to me as to the development of parts in relation to the 

 atlantal foramen. A number of observations on the develop- 

 ment and comparative anatomy of the atlas have been made and 

 will be reported in another place. At present I wish to discuss 

 only those which have some bearing upon the occipital region. 



The atlas of the smallest cat embryo studied (10 nmi.) was 

 represented in cartilage by a pair of neural arches, a small cen- 

 trum about the notochord, and the beginnings of the hypochordal 

 arch. The expanded base of the neural arch (lateral mass) 

 sends dorsad in front of the latter a small, blunt atlantal 

 process. The notch between the process and arch lodging the 

 first spinal nerve and vertebral artery, is the first step toward the 

 formation of the atlantal foramen. In the stage represented 

 by the model, the notch is relatively deeper, owing to the in- 

 crease in length of the atlantal process, which is now in the 

 form of a short bar. The base of this bar, anteriorly, partici- 

 pates in the articulation with the occipital condylar surface; its 

 dorsal extremity is free. The boundaries of the foramen are 



