344 F. H, BDGEWORTH 



from the ventral end of the mesoblast (i. e. myotome down- 

 growth) in the first branchial arch. As stated above, I find it 

 to be developed from the ventral end of the first branchial 

 muscle-plate. 



Behind this muscle are two others, also longitudinal in 

 direction — the Subarcualis rectus v. and the Cleido-branchialis. 

 The latter was first described by Fiirbringer under the name 

 Coraco-branchiales, of which he said five are present, passing 

 from the shoulder-girdle to the five Ceratobranchials, The 

 fifth is the broadest and some of its fibres are attached to the 

 skull. The others are slender. Greil described the development 

 of these muscles as follows. The Subarcualis rectus v. is 

 developed in a 17-8 mm. embryo by forward growth from the 

 mesoblast (i.e. myotome dow^ngrowth) of the fifth branchial 

 arch. Its anterior end becomes attached to the ventral ends 

 of the branchial bars. The Cleido-branchialis is derived from 

 a process of the mesoblast of the fifth branchial arch which 

 grows forwards, separating into three or four pointed extremities 

 (' Zipfel ') which reach the ventral ends of the branchial bars. 

 It forms an aberrant band of muscle which grows in the same 

 direction, but is separate from the Subarcualis rectus v. and 

 gains a secondary relation to the shoulder-girdle. 



I find that these two muscles are developed from a single 

 primordium which appears in a 13-5 mm. embryo (Text-fig. 16) 

 as a slight forward growth from the junction of Levator v., 

 and Transversus ventralis v. This primordium extends 

 forwards, reaching the antero-posterior level of the third 

 branchial arch in a 14 mm. embryo and that of the second 

 branchial arch in a 16 mm. embryo (Text-fig. 28). In the last- 

 mentioned stage its hinder part has increased in vertical depth 

 and its postero-inferior angle is attached to the Cleithrum. 

 In a 28 mm. embryo (Text-fig. 29) the primordium has fully 

 separated into the muscles it forms, viz. the Subarcualis 

 rectus V. passing from the fifth to the first Ceratobranchial, 

 and the Cleido-branchialis. This latter muscle is posteriorly 

 attached to the ventral surface of the Cleithrum and separates 

 into fasciculi which, passing dorsal to the Subarcualis rectus v., 



