MUSCLES OF CERATODUS 327 



third myotome as myotome x of Fiirbringer's classification. 

 The explanation of the difference of opinion is that Sewertzoff's 

 embryo was one in which the variable nerve x was absent. 

 The variation certainly occurs, e.g. in an embryo of 20 mm. 

 Nerves x, y, z were present ; in one of 26 mm. (vide Text- 

 fig. 26) there were only nerves y and z. I therefore follow 

 Greil's nomenclature. Atrophy of myotomes takes place from 

 before backwards, as stated by Sewertzoff. Thus in a 20 mm. 

 embryo only a few fibres of myotome v persisted, in one of 

 24 mm. myotome v has altogether disappeared and also the 

 ventral part of myotome w. 



Coraco-hyoideus and Genio-coracoideus. Greil 

 stated that the Coraco-hyoideus is developed from downgrowths 

 of the third to the sixth myotomes, i.e. myotomes x, y, z, a. 

 These downgrowths separate from the myotomes above, fuse 

 together, and form the Coraco-hyoideus, which extends from 

 the shoulder-girdle to the hyoid bar. The primordium of the 

 Genio-coracoideus separates from the ventral edge of the 

 third myotome (i. e. foremost) constituent of the Coraco- 

 hyoideus in a 13-9 mm. embryo, and fuses with its fellow, 

 forming a median muscle which elongates forwards to the jaw 

 and backwards. The posterior end forks right and left, and in 

 a 17-8 mm. embryo — the latest stage investigated — reaches the 

 antero-posterior level of the first branchial arch. This method 

 of development of the anterior constituent of the hypobranchial 

 spinal muscles is not a usual one, and the initial stages were 

 not depicted. The first figures given, i. e. Nos. 424-9, show 

 the Genio-coracoideus already developed as a median muscle, 

 partly in front of and partly underlying the anterior ends of 

 the Coraco-hyoidei. 



I find that in a 9-5 mm. embryo (Text-figs. 1-5) the peri- 

 cardium extends forw^ards to the hyoid segment, and its anterior 

 end is ventral to the hinder edge of the thyroid outgrowth. 

 The primordium of the hypobranchial spinal muscles lies 

 laterally to the pericardium, and, as this lessens in size, approxi- 

 mates to its fellow. In front of the pericardium the two columns 

 come together and lie beneath the thyroid. The anterior part 



z 2 



