HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 149 



distance beyond the ventral border of the ganghon. In older 

 stages the nerve lies near the medial surface of the ganglionic 

 mass from which it gradually becomes separated until it lies 

 free along the medial side of the ganglion; the two are then con- 

 nected by the so-called short root of the ganglion. 



The notochord at this stage is slender, and slightly wavy in 

 its anterior portion, and has a small cluster of cells attached to 

 its tip, as in previously described instances. 



10-mm. embryo (6) 



Although the measurements for this embryo were the same 

 as for the preceding specimen 'a,' its developmental conditions 

 are somewhat more advanced. The N. trochlearis could here 

 be found for the first time. It extends forward from the brain 

 toward the N. oculomotorius, which it has not yet crossed, and 

 it is still some distance from its muscle. Even after assiduous 

 search, it could not be found in the preceding embryo. 



The premandibular somites have suffered extensive changes. 

 On one side the wall has completely broken down and the cavity 

 has disappeared. The only mark of its former existence is an 

 area of sparsely distributed mesenchyma conforming somewhat 

 to the earlier outline of the cavity. On the other side parts of 

 the somite wall are still seen intact in sections cutting the inner 

 end of the M. rectus superior. Laterally the wall becomes more 

 broken down, and the cells grade insensibly into the general 

 mesenchyma. The medial wall of the first head cavities is thus 

 the last to break down, and this is probably due to the fact that 

 this side is least affected by the disturbing influence of the devel- 

 oping eye-muscles. 



11-mm. embryo; sagittal series; reco7istruction: figure 22 



The abducent muscles have made decided progress. They 

 are now well differentiated and distinct muscles situated close 

 to the posterior surface of the ciliary ganglion, medial and ventral 

 to the ophthalmic division of the N. trigeminus. The cells of 

 the M. retractor oculi have continued their movement forward 



