HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 155 



anterior to the entrance of the optic stalk into the eye-ball. The 

 inner ends of this muscle and the M. rectus medialis are there- 

 fore in close proximity. Together with the M. obliquus inferior 

 they form a second group, at this stage, such that in sagittal 

 sections the M. obliquus inferior is ventral and anterior, the 

 M. rectus medialis dorsal and posterior, and the M. rectus inferior 

 also dorsal and immediately posterior to the second. 



With the exception of the M. retractor oculi which is more 

 complex in form than the rest, the eye muscles have virtually 

 attained their definitive positions, the establishing of their origins 

 alone remaining. 



The M. retractor oculi of the adult Chelydra is of great breadth 

 and length as compared with the other eye muscles, and presents 

 two divisions: a lateral (or external) portion, which arises from 

 the base of the posterior edge of the interorbital septum; and a 

 medial (or internal) portion, the origin of which extends from 

 the origin of the lateral portion, forward, for some distance along 

 the ventral margin of the interorbital septum. The medial por- 

 tion is developed, as previously remarked, from the sheet-like 

 down-growth of the posterior edge of the original muscle, and 

 it is inserted along an area on the eye-ball partially surrounding 

 the optic nerve. The lateral portion is narrower, and is inserted 

 on the posterior surface of the eye mediad of the insertion of the 

 slender M. rectus lateralis. In a lateral view of the eye, the 

 M. rectus lateralis may be seen passing up dorsally along the 

 posterior surface of the eye-ball and crossing the external portion 

 of the M. retractor ocuU. 



The conditions in Chelydra are essentially as given by Bojanus 

 for Testudo europaea, and by Hoffmann for other chelonian repre- 

 sentatives. 



SUMMARY 



The head somites. In embryos of Chelydra serpentina, three 

 prootic head somites are developed on each side. 



The first head somite arises as a lateral outgrowth of ento- 

 dermal cells from the antero-dorsal wall of the foregut. The 

 outgrowing cells form a stalked, compact, and more or less irregu- 



