350 



F. H. EDGEWORTH 



the two downgrowths form together a muscle-plate dorsal to 

 the oesophagus. The Sphincter oesophagi et laryngis is thus 

 developed from two constituents — a ventral derived from a 

 backward extension of Transversus ventralis v. and a dorsal 

 derived from the downgrowths of myotomes x and y. 



The only changes which take place between this condition 

 and the adult form (as determined by transverse sections) is 

 that the Sphincter is completed by medial and lateral spread of 

 its dorsal constituents, and it extends a little farther back- 

 wards in the mid-ventral line so as to underlie the larynx. 



Text-fig. 27. 



dowrL-growbh. 

 from. Mx 



N.intest X 



Trans, vent.v 



oesoph 



Embryo 28 mm., transverse section. 



The Transversus ventralis v. is said by Greil to be innervated 

 by the N. ultimus vagi, i.e. the nerve of the fifth branchial 

 arch. My observations confirm this, and I can add that no 

 additional branch of the vagus is developed in later stages 

 for its hinder part, i.e. the ventral part of the Sphincter 

 oesophagi et laryngis. This is in harmony with its develop- 

 ment. The dorsal part of the Sphincter — developed from 

 myotomes x and y — is innervated by branches of the Ni. occips. 

 X and y, or by the latter only when there is no N. occip. x. 



Wiedersheim (1904) described the adult condition of the 

 Sphincter oesophagi et laryngis in all three Dipnoi,^ and 



^ He gave figures of Protopterus and Lepidosiren, but not of Ceratodus. 



