DEVELOPMENT OF THE SKULL OF EMYS 701 



extends forward through the crista and opens on its anterior 

 surface into the fenestra hypoijhyseos. 



In passing through the crista sellaris the chorda tapers from the 

 uniform diameter which it exhibits posteriorly to a rounded point. 



As might be expected' in a part undergoing rapid degeneration, 

 the anterior end of the chorda exhibits considerable variations in 

 its relations to the crista sellaris. In several specimens it was 

 bent in a dorso-ventral direction within the crista so that the 

 anterior portion lay in a plane dorsal but parallel to the posterior 

 portion. In most of the embryos studied the crista was com- 

 pletely perforated by the canal for the chorda, but in one young 

 embrj'o the canal terminated within the cartilage. In another 

 embryo onlj^ slightly younger than that modelled, the anterior 

 end of the chorda projected freely into the fenestra hypophyseos 

 while in the model, as in most of the specimens studied, the end 

 of the chorda wqs flush with the anterior surface of the crista. 



At the extreme caudal end of the condylus in the embryo 

 modelled, the chorda is completely svuTounded by cartilage, but 

 in a very slightly younger embryo the condylus at its extreme 

 caudal end exhibits a U-shape in cross section quite similar to 

 that of Lacerta; that is, the chorda was surrounded only ventrally 

 and on. the two sides with cartilage (fig. 15). Later, however, 

 the chorda is completely surrounded. Immediately surrounding 

 the chorda the cartilage becomes excavated to form a cup-like 

 depression, the central cavity, into which the dens epistrophei 

 fits. The chorda passes from the anterior face of the epistropheus 

 into the posterior face of the condyle, forming a condylus anularis 

 characteristic of the Chelonia. The ventral sui'face of the con- 

 dyle articulates with the atlas which projects ventrally to it. 



In contrast with the condyle of Lacerta, as Gaupp has shown, 

 that of Emys does not exhibit the two processes, one on each side 

 of the chorda dorsalis, but rather a single ring-like process around 

 the chorda dorsalis. In view, then, of the embryonic condition 

 in Lacerta, the derivation of the mammalian condyles from those 

 of reptiles is not improbable. In Chelone, according to Gaupp, 

 the chorda enters the condyle from the ventral side so that. 



