154 CHARLES EUGENE JOHNSON 



relation to the oculomotor nerve and ciliary ganglion remains 

 the same. Its end of origin lies in close relation with the abducent 

 muscles. In sagittal sections the Mm. rectus superior and re- 

 tractor oculi are situated dorsally, forming respectively the 

 anterior and posterior members of the group, and the M. rectus 

 lateralis forms the ventral member. 



The M. rectus medialis {red. med.) is not soon separated from 

 its twin muscle the M. rectus inferior {rect. inf.), and in the 

 Chrysemys embryo there is still a slender connection between 

 them. The M. rectus mediahs has worked its way dorsally to 

 become inserted on the antero-medial surface of the eye-ball. 

 In the course of this migration the M. rectus inferior has been 

 drawn anteriorly and lies close against the ventral surface of 

 the optic stalk, below which it has found its insertion. The 

 area of connection of the M. rectus inferior with the M. rectus 

 mediahs extends from about the middle of the medial side of 

 the latter to its free end. The condition found in the Chelydra 

 embryo shows that a like relationship of these muscles has existed, 

 but the two muscles are now separate from each other. Evidently 

 the M. rectus medialis, in growing forward toward its insertion, 

 has drawn the free end of the M. rectus inferior further mediad 

 and ventrad. 



On the ventral surface of the M. rectus medialis, where it 

 crosses the optic stalk, there is a small, globular, solid cell-mass 

 (z) for which I am unable to account, unless it may be an acci- 

 dentally separated portion of muscle or nerve tissue. 



The M. obliquus inferior {obi. inf.) has effected its insertion 

 in common with the M. rectus inferior, and has turned about its 

 ventral end in a medial direction, following the change in position 

 of the M. rectus medialis. It has now received its branch from 

 the N. oculomotorius, the nerve entering on its posterior side, 

 a short distance from the insertion. 



In Chelydra the Mm. recti medialis and inferior have just 

 become separated, the separation having taken place at their 

 free ends (origins). The M. rectus inferior has been drawn for- 

 ward in this process, its end of origin having been moved from 

 its proximity to the ciliary ganglion to a position ventral and 



