MOEPHOLOOy OF (RAXIAL :\irSCr,ES IX SOMK VERTKERATES. 263 



the sphincter laryngis and dilatator laryngis (Text-fig. 69) are 

 formed, the latter having a vertical position and passing up 

 outside the vagus. The dilatator has lost this position in 

 15 mm. embryos, and its hind end is attached to the cricoid. 

 This supports the theory of Goppert that the dilatator 

 laryngis of Sauropsida is the homologue of the dorso-laryn- 

 geus. and the sphincter tlio homologue of the sphincter and 

 laryngei of Amphibia. 



In 8 mm. embryos of the ral)bit, cells are prolifei'ated from 

 the coeloraic epithelium in the thi-ee branchial segments 

 (Text-fig. 80) ; they spread upwards internal to the branchial 

 aortic arches. In 5 mm. embryos they have spread a little 

 round the pharynx in the second and third branchial segments 

 (Text-figs. 85, SQ), but in the first they exist only lateral to 

 ifc. In 6 mm. embryos the splanchnic inesoblast forms a con- 

 tinuous sheet dorsal to the pharynx in the second and third 

 segments, but in the first it is still only lateral to it. In 

 7 mm. embryos it is present there also, more probably as a 

 result of forwai-d extension from the second branchial seg- 

 ment than of upward extension in the first segment. This 

 continuous splanchnic mesoderm sheet is continuous behind 

 with that round the oesophagus (Text-fig. 90), which is 

 formed in a similar way. 



The stylo-pharyngeus is formed in the splanchnic meso- 

 blast of the first branchial segment; it is first visible in 

 7^ mm. embryos and gains an attachment to the hyoid bar 

 (Text-fig. 93). The pharyngeal constrictor is also formed in 

 the three branchial segments, though, perhaps, vide supra, 

 derived from cells of the second and third segments onlv. 

 The Anlage of the lai-yngeal muscles is also visible in 7^ mm. 

 embiyos in the third branchial segment. The later develop- 

 ment of the laryngeal muscles, in man, has been traced by 

 Frazer, Avho says that they are developed in the third bran- 

 chial segment from the ventral part of the layer of cells 

 round the pharynx and larynx ; of this laryngeal portion 

 "the formation of the arytenoid seems to convert the dorsal 

 hinder part into the crico-arytenoidens posticus, the upper 



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