.MUKl'HUL(_)(iY OF CIJANIAL ML'SCLES IX SUM t; VEKTEBliA'l'KS. 257 



stvictors, iuterbrauchials, and arcuales dorsales external to 

 them. The lower ends of the superficial constrictors extend 

 downwards external to the coraco-branchiales, but such down- 

 growths do not appear to be homologous with the traus- 

 versi ventrales or inferior portions of the obliqui ventrales 

 of Teleostomij Ceratodus, and Amphibia. 



In Sauropsida, rabbit, and pig embryos the portions of 

 the branchial myotomes next abov^e the Anlagen of the hypo- 

 branchial cranial muscles, and also the lower ends where these 

 Anlagen are not formed, atrophy. 



Levatores arcuiim branchialum are developed from the 

 upper ends of the branchial myotomes in Teleostomi, Cera- 

 todus, and Amphibia, bnt are not developed in Scy Ilium, 

 Sauropsida, rabbit, and pig. The method of development of 

 the trapezius — apparently a homologous muscle throughout 

 these vertebrate groups — is intimately related to these differ- 

 ences. It is developed in Teleostomi and Amphibia from the 

 fourth, in Ceratodus from the fifth, levator, i.e. from the 

 penultimate or ultimate levator^; whereas in Scyllium, 

 Chrysemys, Callus, and rabbit, it is formed from the upper 

 ends of the branchial myotomes — five in Scyllium, four in 

 Chrysemys, two in Callus, and three in the rabbit. 



In view of the facts that in tScyllium the subspinalis and 

 interbasales, developed from trunk-myotomes, are attached 

 to the pharyngo-branchials, and that the trapezius is inner- 

 vated only by the Xlth — the most posterior of the vagus 

 roots — even though a constituent from the glossopharyngeal 

 (first branchial) segment takes part in its formation, it is 

 probable that the absence of levatores and associated method 

 of development of the trapezius in Scyllium, Sauropsida, and 

 rabbit are secondary phenomena, and that the primary con- 

 dition is a series of levatores formed from the uppermost 

 portions of the branchial myotomes. This theory would also 

 afford an explanation of the curious fact that whereas the 



' In Teleostonian emljryos the trapezius is developed from the upper 

 edge of tlie levator, in Ceratodus and Amphibia from its external 

 surface. 



