HEAD-MUSOLES IN GALLUS AND OTHER SAUROPSIDA. 521 



In Splienodon there are similarly four vertebras visible (text- 

 fig, 26, p. 529). Ill tlie other Sauropsida investigated only- 

 three were visible — probably due to a dropping out, in deve- 

 lopment, of the most anterior. In Tropidonotus, for instance, 

 the first two hypoglossus roots pass out together, i. e. the 

 irvening vertebra is not developed, even as precartilage. 

 5tal processes or cranial ribs were only seen in Spheuodpn 

 (text-fig. 26, p. 529), and then only over the hindermost coalesc- 

 ing vertebra; these persist, forming on each side the structure 

 known as the '' proatlas." ^ In this embryo chondrification 



ir>f R inf temp t mass an 



Text-fig. 14. — Sagittal section tlivougla an embryo of Gallus 

 at beginning of seventh day. (For explanation of lettering see 

 p. 555.) 



had already taken place in the basis cranii, so that possibly in 

 an earlier stage a greater number of costal processes was 

 present. A similar explanation probably holds for the 

 "proatlas" of the Alligator. 



' Howes and Swinnerton did not see the posterior part of the basis cranii 

 in stage P, and by stage Q the evidence of coalescence of vertebrse has disap- 

 peared. They conseqnently did not express any opinion as to the nature of 

 the " proatlas." 



