HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 127 



and Corning compares these two divisions with Hoffmann's find- 

 ings for Acanthias, and would call them respectively the Mm. 

 obhquiis inferior and rectus inferior. Both are connected with 

 each other for some distance from their point of origin on the 

 somite wall. 



The dorsal anlage has not, at this stage, advanced very far 

 and Corning states in regard to it that he was able to establish 

 only that it divides into parts to which the upper branch of the 

 N. oculomotorius is given off, and that these parts give rise to 

 the M. rectus superior and probably to part of the M. rectus 

 medialis. The development of these muscles was followed no 

 further by Corning. 



As before mentioned, the M. obUquus superior, according to 

 this author, arises from the dorsal part of the trigeminal or max- 

 illo-mandibular muscle anlage. On plate 6, figure 33, he pic- 

 tures the M. obliquus superior as an uninterrupted dorsal exten- 

 sion of the trigeminal muscle-mass, ending a short distance above 

 the ophthalmic division of the trigeminal nerve. No structure 

 answering to the second head somite of other authors is thus 

 recognized. 



The abducent muscles are derived from the earlier mentioned 

 third head somite. Corning does not associate the two muscles 

 of this group with any two divisions of the third head somite, 

 and does not give figures of the last named structure, because, 

 he states, it agrees in every respect with the figure presented by 

 Oppel for Anguis fragilis. From the conditions found in a late 

 embryonic stage of L. vivipara, however, he remarks that, for 

 the musculature innervated by the abducent nerve, one has to 

 distinguish between two origins: a posterior one, from which 

 proceeds the greater part of the M. retractor oculi; and an anter- 

 ior one, from which the M. rectus lateralis arises. This is because 

 he finds that part of the abducent muscle-mass becomes attached 

 posteriorly to the trabeculae, and part, passing medially between 

 the hypophysis and the trabeculae, becomes attached to the 

 bony plate separating the hypophysis from the oral cavity. 



Filatoff's observations on the oculomotor muscles agree with 

 Coming's, but are likewise incomplete. On plate 10, figure 28, 



