354 F. H. EDGEWORTH 



is developed as a median ventral outgrowth- of the endoderm 

 in the last (fifth) branchial segment and anterior end of the 

 oesophagus, and subsequently shifts backward relative to the 

 sixth gill-cleft so as to be situated entirely in the oesophageal 

 region. During this period the larynx increases in size, though 

 to a far less extent. Thus in the 12 mm. embryo the epithelium 

 of the larynx is continuous with the endoderm above over 

 a length of 86 fi from front and back ; in the 20 mm. embryo 

 over a length of 120 /i. The trachea begins to be formed in the 

 13-5 mm. embryo as a posterior outgrowth of the larynx. 



The larynx of Ceratodus is thus developed in the same 

 position as in Menopoma, but unlike that of Menopoma it 

 subsequently shifts backwards. The backward extension of 

 Transversus ventralis v, is in relation to this. There is no 

 similar extension of Transversus ventralis iv. in Menopoma. 



Oesophageal and Laryngeal Muscles.— The only 

 statement that Greil made in regard to these muscles is that 

 in a 13-9 mm. embryo the free mesoderm cells increase in 

 numbers round the oesophagus. These cells are chiefly of 

 paraxial origin. They form a mantle round the gut and become 

 for the most part smooth muscle-cells. 



I find that a Constrictor oesophagi and a Constrictor laryngis 

 are present in the adult, and that they are developed from 

 cells which are budded off from the epithelium of the pericardio- 

 peritoneal ducts in an 11 mm. embryo (Text-fig. 10). These 

 cells increase in numbers and spread round the larynx and 

 oesophagus, and in a 16 mm. embryo form the above-mentioned 

 muscles, which are better developed in a 20 mm. embryo from 

 which the figures are drawn (Text-figs. 22-4). The Constrictor 

 oesophagi extends from the level of the anterior edge of the 

 larynx to the posterior end of the oesophagus. In the laryngeal 

 region its fibres are interrupted ventrally and pass towards the 

 epithelium of the larynx through the fibres of the Constrictor 

 laryngis. Behind the larynx the fibres encircle the oesophagus. 

 The anterior part of the Constrictor oesophagi thus acts 

 additionally as a dilatator of the larynx, but this part is con- 

 tinuous with the posterior part, and remains so up to the adult 

 state. A separate Dilatator laryngis is not formed. 



