MUSCLES OF CERATODUS S31 



the Coraco-liyoideus. The Coraco-hyoideus hes laterally to the 

 pericardium, and its anterior end is 56 /x behind the thyroid. 

 The Genio-coracoidei form a A -shaped structure. The anterior 

 median part is ventral to the thyroid : it extends behind 

 this for 104 /x, diverging into two lateral ends which lie beneath 

 the anterior ends of the Coraco-hyoidei. The Genio-coracoidei 

 consist of yolk-laden cells, the Coraco-hyoidei of muscle-cells. 

 In an 11 mm. embryo th« anterior end of the Genio-coracoidei 

 extends a little farther forwards — in front of the thyroid, and 

 in a 12 mm. embryo reaches Meckel's cartilages. Its cells become 

 transformed into muscle-cells in a 13 mm. embryo. 



The Genio-coracoidei extend slowly backward, diverging into 

 right and left halves. These ;reach the level of the third 

 branchial arch in a 20 mm. embryo, and become attached to 

 the lateral edges of the median cartilage — called ' sternum ' 

 by Greil — which forms the ventral constituent of the cartila- 

 ginous shoulder-girdle, in the 28 mm. embryo. 



The above may be surriiiiarized in the statement that the 

 primordium of the hypobranchial spinal muscles extends 

 forwards, laterally to the pericardium, and in front of this 

 joins its fellow. It then separates into Genio-coracoideus and 

 Coraco-hyoideus. The Genio-coracoidei, in contact with each 

 other from the first, form a median structure which extends 

 forwards to the jaw and backwards to the shoulder-girdle. 



Fiirbringer stated that the Genio-coracoideus of the adult 

 form extends from the mandible to the shoulder-girdle (coracoid 

 and clavicula) and has one tendinous inscription on its inner, 

 i. e. dorsal surface. This, he said, gives rise to the idea that it 

 originally consisted of two myomeres, but this structure may 

 be secondary. This latter supposition is confirmed by the fact 

 that in a 28 mm. embryo (in which the posterior end of the 

 muscle has reached the ' sternum ') there are no inscriptions 

 in the muscle. The same explanation applies to the three 

 inscriptions depicted in the muscle by Maurer. 



Innervation. — Fiirbringer stated that the Coraco- 

 hyoideus and Genio-coracoideus are innervated by the Plexus 

 cervicalis, i. e. Nervi spinales x, y, z, or y, z. 



