HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 147 



backward. The latter vein extends along the outer surface of 

 the larger caudally directed part of the muscle-mass, which gives 

 rise to the M, retractor oculi. There can be little doubt that 

 the relation to the veins is a considerable factor in hindering 

 the forward migration of the abducent muscles at this stage, 

 and in directing them outward. In the model the Mm. rectus 

 laterahs and retractor ocuh are not sharply differentiated from 

 each other, but in sagittal sections they are well defined, owing 

 to the difference in the direction of their cells, and to the fact 

 that the M. rectus lateralis is more compact. The cells of the 

 M. retractor oculi have migrated forward, extending somewhat 

 dorsally and laterally over the inner end of the M. rectus lateralis, 

 so that in sections the latter muscle, at this end, appears as a 

 heap of cells crowded to a greater or less degree ventrad of the 

 anterior end of the M. retractor oculi (fig. 17). A crossing of 

 the two muscles is thereby begun. 



The abducent nerve can now be traced to the M. retractor 

 oculi which it penetrates on the median side of its tapering caudal 

 end. 



The M. ohliquus superior has greatly increased in length and 

 extends forward in a gentle curve above the dorso-medial surface 

 of the eyeball. Its structure is most compact at about its middle, 

 where it is slightly thicker; it tapers anteriorly and also pos- 

 teriorly to where it is continuous with the remaining ventral 

 portion of the original somite. This latter part retains its pre- 

 vious relation to the N. trigeminus and to the dorsal mass of 

 the maxillo-mandibular musculature, but is somewhat reduced 

 in extent through its contributions to the M. obliquus superior. 

 Its cell-structure is more open than that of any of the differ- 

 entiating muscle-masses. The dorso-ventral direction of its dis- 

 tinctly spindle-formed cells suggests that they are moving ven- 

 trally to fuse with the maxillary and mandibular musculature; 

 but the latter gives no evidence of having received additions 

 from this source, with the exception, possibly, of the part directly 

 adjacent. In figure 16 it would appear as though a dorsal group 

 of cells of the maxillary portion of the muscle-mass were formed 

 entirely by cells from the mass x, but in reality it is but a part 



