MORPHOLOGY OF CRANIAL MUSCLES TN SOME VERTEBRATES. 213 



fold (Text-fig. 40), and form in the latter stage (Text-fig. 38) 

 anteriorly a levator liyoidei inserted into the upper end of the 

 hyoid bar, and an interhyoideus, and posteriorly a continuous 

 ventro-dorsal sheet Cjvd in the operculum. In stage 42 the 

 Anlage of the hyoraaxillaris ligament is cut off from the 

 upper edge of the intei-hyoideus and spreads forwards to the 

 hind end of Meckel's cartilage (Text-fig. 41). In stage 42 

 (Text-fig. 41) the hyoid bar extends upwards and inwards 

 towards the under surface of the pro-cartilaginous tract con- 

 necting the parachordal plate with the auditory capsule, 

 forming the hyomandibula, the original hyoid bar forming 

 the ceratohyal and hypohyal. 



The upper part of the originally pi'O-cartilaginous hyoman- 

 dibula chondrifies : the lower forms a fibrous tract connectin"- 

 its outer end with the upper end of the ceratohyal (Text-fig. 

 50). In stage 48 a downgrowth occurs from the outer edge 

 of the auditory capsule, external to the hyomandibular branch 

 of the Vllth (Text-fig. 47), and becomes sepai'ated, forming a 

 cartilage abutting against the outer end of the hyomandibula 

 (Text-fig. 50), and a second more dorsally situated piece is 

 subsequently cutoff from the auditory capsule (Text-fig. 50).^ 

 The insertion of the levator hyoidei into the upper end of the 

 ceratohyal is preserved in the oldest embryo examined, but 

 is not present in the adult (Ruge) ; it is, however, retained in 

 Protopterus (vide description by Ruge). 



In Necturus Miss Plate stated that the "hyoid mesothelial 

 tissue" (here interpreted as *•' myotome ") divided into a 

 dorsal digastricus, Avhich "is ultimately connected wdth the 

 posterior extremity of the mandibular bar by a long tendon" 



' This description of the liyomandilmla coincides with and amplifies 

 that of K. Fiirbringer. The cartihige (or cartilages) cut off from the 

 auditory capsule is probably, from its relation to the hyomandiljiilar 

 branch of the Vllth, that described by Huxley, Ridewood. Ruge, and 

 Sewertzoff as the "hyomandibula." From the descriptions given it 

 would appear probable that no part of a true hyomandibula is preserved 

 in the adult, and that {vide Ridewood) the cartilages cut off from the 

 auditory capsule are varialjle. 



