HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 153 



outgrowth has barely started and cannot be seen in the model). 

 It is the further development of this sheet of muscle which results 

 in the very broad median portion of the M. retractor oculi in 

 the adult turtle, where it forms an extensive insertion, partially 

 encircling the optic nerve at its entrance into the eye-ball. 



As the two abducent muscles, in approaching their insertion, 

 have so turned as to he approximately transverse to their former 

 direction, the abducent nerve reaches them at right angles to 

 their long axes. It enters the M. retractor oculi directly on its 

 postero-ventral side. Immediately before penetrating this mus- 

 cle, it gives off a large branch which follows closely the ventral 

 surface of the M. retractor ocuh, on its way to the M. rectus 

 lateralis which it enters from the dorsal side at about the middle 

 of its length. 



In Chely!lra at this stage a slender nerve comes from the tri- 

 geminal ganglion at the root of the nasociliary nerve, and pene- 

 trates the M. retractor oculi in a direct line with the ciliary gan- 

 glion. This evidently is the so-called long root of the ganglion. 

 In the Chrysemys embryo in which it has already connected 

 with the ganglion, it arises from the nasocihary nerve more dis- 

 tally (l.r.), and passes down between the Mm. rectus superior 

 and retractor oculi, parallel to the N. oculomotorius. 



The M. ohliquus superior. This muscle {ohl. sup.) has greatly 

 increased in length. It is fixed at its insertion and is about to 

 become fixed at its origin. Its former position corresponds with 

 its inserted end, from which it extends ventrally and anteriorly, 

 following closely the surface of the eye-ball. As it reaches the 

 nasociliary nerve where this leaves the anterior surface of the 

 eye, it makes a slight bend, passing dorsad of the nerve and 

 continuing medially. In Chrysemys the muscle is less advanced 

 and is shorter, but it has the same direction and position with 

 regard to the eye-ball. The N. trochlearis (n. troc.) has reached 

 the M. obliquus superior a short distance from its insertion, 

 penetrating its posterior edge. 



The oculomotor group. The M.- rectus superior (rect. sup.) 

 has changed but slightly, having merely straightened somewhat, 

 and grown further forward, and established its insertion. Its 



