MORPHOLOGY OF CRANIAL MUSCLES IX SOME VERTEBRATES. 221 



Futamura's account of tlie development of the facial and 

 platysnia muscles is that their Anlage separates, as stated 

 above, from the outer aspect of the common facial Anlage ; 

 in thirty-one to thirty-four day embryos it spreads aborally 

 almost to tlie shoulder regiou, forming the platysma colli, and 

 in six-week embryos also forwards, in two directions sepa- 

 rated by the outer ear, formiug a platysma occipitalis and a. 

 facial portion. The latter passes forwards over the edge of 

 the lower jaw to the forehead, eyelid, upper lip, and temporal 

 region, separating into a deep and a superficial layer. 



In 3 mm. embryos of the rabbit the hyoid myotome is con- 

 tinuous below Avith the epithelium lining the hyoid section of 

 the cephalic coelom (Text-fig. 77). In 4 mm. embryos the 

 wails of the cephalic coelom have come together, and the 

 Anlage of the inteihyoideus is formed from its epithelial wall ; 

 each half is continuous laterally with the lower end of the 

 myotome (Text-fig. 81). In 7^ mm. embryos the myotome is 

 partially separated into an upper and a lower part : the former 

 is the Anlage of the posterior digastric, stylohyoid, and stape- 

 dius, the latter that of the anterior digastric (Text-fig. 89). In 

 embryos between 8 and 11 mm. the hyoid bar is formed,^ and 

 in its whole extent simultaneously, so that there is no stage 

 in which the Anlage of the posterior digastric, stylohyoid, and 

 stapedius is inserted into the upper end of a hyoid bar which 

 has not yet extended up to the auditory capsule. In 9 mm. 

 embryos (Text-figs. 92 and 93) the lateral edge of the inter- 



^ The ■■ body of the hj'oid "" in the rabbit is formed from the ventral 

 ends of the hyoid and first branchial bars. No basihyal nor basi- 

 branchial is developed. In 9 and 13 mm. embryos a column of cells — 

 the remains of the glosso-thyi-oid dnct — extends downwards from the 

 foramen cseciim in the middle line between the hyoid and first branchial 

 bars. Antero-posterior fusion of the ventral ends of these l)ars to form 

 the body of the hyoid, and the commencement of formation of a joint 

 between the body and the first branchial cornua, take place in embryos 

 between the stages of 13 and 17 mm. Chondi-ification takes place later 

 in the " body " than in the hyoid and first branchial comua. These 

 findings confirm the suggestions of Parsons ; in the rabbit, however, 

 there is no basihyal. 



