244 r. H. EDGEWORTII. 



tlie wall of the pericnrdium, the second and third arising as a 

 single muscle from the wall of the pericardium in the region 

 where the mesothelinm of the second vagus arch unites with 

 the pericardial wall. Above these muscles are found the gill- 

 muscles, and dorsally the three levatores arcuuni. 



This would mean, according to the theory which was 

 suggested above, that the interarcuales ventrales I, II, and 

 III ai-e formed from the ventral ends of the first, second, 

 and third bi-anchial myotomes, the Anlagen of the gill- 

 muscles above these, and the three levatores from the upper- 

 most portions. 



There are three other bmnchial. muscles in Necturiis which 

 were not mentioned by Miss Piatt — the transversus ven- 

 tralis IV,^ fourth- levator arcuum, and the trapezius.' In 12 

 mm. embryos there is present a fourth branchial myotome 

 serially homologous with the first, second, and third (Text-figs. 

 51, 52). In 13 mm. embryos this has separated from the 

 cephalic coelom and divided into a fourth levator and lateral 

 half of a transversus ventralis IV (Text-fig. 53) ; in 147* mm. 

 embryos the lower half of the transversus ventralis IV has 

 spi'ead inwards dorsal to the cephalic coelom and below the 

 developing larynx to meet its fellow in the middle line (Text- 

 fig. 56).^ No interarcualis ventralis IV is developed. Trans- 

 versi ventrales are not developed in the first three arches.'' 

 The trapezius is proliferated from the outer surface of the 

 fourth levator in 16 mm. embryos. 



In Triton cristatus the events are similar; an interarcu- 

 alis ventralis IV is developed, in correlation with the formation 

 of the fourth branchial bar. The interarcuales venti-ales II, 

 III, and IV become divided into the muscles cjilled snb- 



' The foiirtli iiharyiigo-liriinchialis of Wilder; tlie hyo-pharynyeiis of 

 Giipjiert. 



^ The fourth levator and traj)ezius were described l)y Mivart ; tlie 

 latter, in the tevminolugy of Fiirbvinger, is a dorso-scapiilaris. 



' Tills coiifiniis the opinion of GiJppert that his hyopharyiigeus is 

 not formed by fusion of transversi ventrales III and IV, Init is only a 

 transversus ventralis IV. 



