152 CHARLES EUGENE JOHNSON 



The M. obliquus inferior at this stage is situated at the outer 

 side of the M. rectus mediaHs, extending sUghtly beyond it anter- 

 iorly. Its posterior end Hes transversely across the slit-like 

 remains of the optic groove. Like the two preceding muscles, 

 it is also stoutly club-shaped, with its broad end anterior. On 

 neither side of the embrj^o is there as yet any branch from the 

 N. oculomotorius to this muscle, which is the only one of the 

 group that has not received its nerve supply. 



Chelydra embryo with carapace of 8.5 mm. 



Figure 23 is a reconstruction of the eyeball with its associated 

 muscles and nerves, of a 9-mm. Chrysemys marginata belonging 

 to the Harvard Embryological Collection (Series 1085). It rep- 

 resents a stage in development of the eye muscles considerably 

 advanced over that of the preceding stage of Chelydra. A recon- 

 struction of these parts of an embryo Chelydra serpentina with 

 a carapace length of 8.5 mm. shows a somewhat more advanced 

 condition, but one directly derived from a stage such as shown 

 for Chrysemys. As a drawing of this model is not available, 

 Mr. Oliver's excellent illustration of the Chrysemys model will 

 adequately serve to elucidate the description for Chelydra. 



The abducent group. The M. rectus lateralis {rect. lat.) has 

 rotated in such a way that it lies approximately transverse to 

 the long axis of the body, in a horizontal plane. It is the most 

 slender of the eye muscles. Its laterally directed end narrows 

 somewhat as it passes outward to become inserted on the pos- 

 terior surface of the eye-ball in the equatorial region. 



The M. retractor oculi {retr. oc.) is here a short massive mus- 

 cle, lying upon the dorsal side of the M. rectus lateralis, and 

 crossing it in an antero-lateral direction, so that an x-shaped 

 figure is formed. As it passes onto the eye-ball, which it reaches 

 somewhat further mediad than the insertion of the M. rectus 

 lateralis, a broad sheet of fibers pushes out from along the postero- 

 ventral border of the muscle and creeps antero-ventrally down 

 over the surface 'of the eye-ball, towards the junction of the 

 latter and the optic stalk. (In the embryo of Chrysemys this 



