286 F. H. EDGEWORTH. 



Alligator, and the genio-hyoid of Gallus, were the only ones 

 found. Otherwise if a muscle is not present in the adult it 

 is not formed during development. 



There are certain instances in which comparative evidence 

 suggests that ancestors probably possessed muscles which are 

 now no longer developed, even as Anlagen. Such are the 

 genio-hyoid of Lepidosteus and Salmo, certain Mm. trans- 

 versi ventrales in Amphibia and Teleostomi, the first two 

 obliqui ventrales in Polypterus senegalus, the hyo-niaxil- 

 laris inSelachii andSauropsida,the levatoresarcuum branchia- 

 lium in Selacliii, Sauropsida, and Mammalia. 



Consideration of the changes whicli take place in the 

 Anlagen of the cranial muscles in the various Vertebrate 

 groups suggests that the most important are those occurring 

 in the myotome of the mandibular segment. In Amphibia 

 and Ceratodus it does not, whilst in Teleostomi, Selachii, and 

 Sauropsida it does divide into parts above and below the 

 palato-pterygoid or pterygoid process of the quadrate. It 

 has been stated above that the embryological phenomena 

 support the view that the second condition has been derived 

 from the first. In the rabbit the quadrate (incus) has no 

 pterygoid process, and the myotome — as in Amphibia and 

 Ceratodus — does not divide into upper and lower parts. 



Changes take place in the Anuran tadpole, in the form of 

 the palato-quadrate bar and in certain muscles in association 

 with the development of a suctorial mouth, i.e. the back- 

 ward elongation of the mandibular muscles, the development 

 of a submentalis and mandibulo-labialis, the origin of the 

 orbito-hyoideus, or of this and the suspensorio-liyoideus, and 

 the partial origin of the first branchial levator from the 

 palato-quadrate bar, the division of the hyo-maxillaris and 

 attachment of one or two of its parts to the palato-quadrate 

 bar. As the condition before these events takes place is 

 very like that of an embryo of Ceratodus or an Urodelan, 

 it would appear probable that the changes are secondary 

 larval ones and not ancestral.^ 



' The difficult question as to the orii^in and nature of the larval 



