HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 159 



When the third head somite has resolved itself into the two 

 unequal but united portions above described, a gradual forward 

 movement occurs. In this the posterior or retractor ocuU portion 

 plays the more conspicuous part, taking the lead and pushing 

 its anterior end up along the median side of the M. rectus later- 

 alis." The M. rectus lateralis portion, in the meantime, has be- 

 come a more compact and better differentiated mass, in intimate 

 association with the antero- ventral end of the M. retractor oculi. 

 When the 11-mm. stage is reached the muscles form two separate 

 and distinct masses of unequal size, lying just ventro-mediad 

 of the ophthalmic division of the N. trigeminus, near the pos- 

 terior side of the ciliary ganglion. Their principal axes have 

 changed from a horizontal direction to a more nearly dorso- 

 ventral one; and the M. rectus lateralis has also turned laterally 

 towards its insertion on the posterior surface of the eye-ball. 

 Its insertion is established in an embryo with a carapace length 

 of 8.5 mm. and after extending shghtly towards its point of 

 origin its final position is reached. 



At this stage the posterior side of the M. retractor oculi has 

 given rise to a broad sheet-hke down-growth of fibers, the free 

 edge of which grows along the surface of the eye towards the 

 junction of the optic stalk and cup; this part becomes the medial 

 division of this muscle as it occurs in the adult. The muscle 

 then begins to grow posteriorly and medially towards its origin. 



The abducent nerve first appeared in the 9-mm. stage. No 

 connection with the abducent muscles can be made out until 

 the 10-mm. stage. Here the nerve has penetrated the retractor 

 portion of the muscle-mass. The abducent nerve arises from 

 the brain by a number of very delicate rootlets, three of which 

 can be counted in the 9-mm. stage; in an 11-mm. embryo six 

 were found. In this stage the M. rectus laterahs has received 

 its nei-ve supply through a stout branch given off from the N. 

 abducens at the point where it enters the M. retractor ocuU. 



The M. obliquus superior. This muscle in Chelydra develops 

 in precisely the same way as described by Filatoff for Emys 

 lutaria, and the process accords with that observed in many 

 other vertebrate forms. The muscle grows forward as a stream 



