MORPHOLOGY OF CRANIAL MUSCLES IX SOME VERTEBRATES. 271 



proliferation of an internjil portion of the genio-liyoid ; other- 

 wise the condition in the larvae is the same. In Alytes the hind 

 end of the genio-hyoid is attached to the third cerato-branchial. 

 In the rabbit the hypobranchial spinal muscles are formed 

 from downgrowths of the first three trunk myotomes m 

 4 mm. embryos (Text-fig. 82). These have separated in 4^ 

 mm. embryos, and form a longitudinal column which extends 

 forwards dorsal to the iuterbyoideus and iutermandibularis, 

 reaching the anterior extremity of Meckel's cartilage in 



8 mm. embryos, and backwards, reaching the area of the 

 anterior limb in 7 mm. embryos. In 13 mm. embryos it 

 has divided into genio-hyoid and f primitive) sterno-hyoid, 

 the adjacent ends of which are attached to the first branchial 

 bar. In 17 nun. embryos the primitive sterno-hyoid has 

 divided into the sterno-hyoid, sterno-thyroid, thyrohyoid, 

 and omo-hyoid. The first trunk myotome, from which the 

 most anterior (jf tiie downgrowths above mentioned takes 

 place, atrophies in 7^ mm. embryos, the second and third in 



9 mm. embryos. 



The Homologies of the Hypobranchial Spinal 

 Muscles. — In Amphibia, Sauropsida, and rabbit, tlie Anlage 

 of the hypobranchial spinal muscles divides into anterior and 

 posterior portions — the genio-hyoid and sterno-hyoid. The 

 former extends from the symphysis of the lower jaws to the 

 basi-branchial or some branchial bar, the latter extending 

 thence to the shoulder-o'irdle or sternum. The division takes 

 place in the neighbourhood of the first branchial bar in 

 Urodela, Sauropsida, and rabbit; in Anuran larvte it is at 

 the level of the third branchial bar. 



In Scyllium, Teleostomi, and Ceratodus, a similar division 

 of the Anlage of the hypobranchial spinal muscles takes place 

 at the level of the hyoid bar; the hind end of the anterior 

 portion, which does not gain any temporary insertion to the 

 hyoid bar, then grows backwards ventral or ventro-lateral to 

 the posterior portion (coraco-hyoideus) and becomes attached 

 to the first (Polypterus, ? species, described by Pollard), or 

 to the second and third (Amia), or third (Polypterus sene- 



