150 THE ANCESTRY OF VERTEBRAT ES 



the epibranchial musculature is formed (DOHRN, 1885, p. 446, 

 HOFFMANN, 1898, p. 265) which in ail other Gnathostomes, 

 and also in rays already, is absent. The greatest number 

 of epibranchial myotomes will be found in hexanch and 

 heptanch Selachians where the number of gill-slits is greatest. 

 Here indeed the epibranchial musculature is best developed 

 (FuRBRlNGER, 1897, p. 416) and so are the occipital nerves 

 supplying them which lie in front of the hypoglossus-roots 

 (v, w, X, according to FuRBRlNGER). 



In the development of the somites from the unsegmented 

 mesoderm a retardation in the head region, correlated with 

 the degree of degeneration of the corresponding myotomes, 

 is to be noticed. The first somite to be differentiated is 

 as a rule the first to develop a permanent myotome. This 

 serves to explain why in the Craniata the development of 

 the somites begins in the neck region and not, as in Amphioxus, 

 with the foremost one (Neal, 1898, p 195). Thus in Acanthias 

 the 3'^'* or 4^*' post-otic somite is the first to develop. As 

 we have seen, this rule has been con^rmed for Petromyzon 

 by KOLTZOFF (1902, p. 318). Here the first post-otic somite 

 is the first to appear. 



The hypobranchial musculature (Musculi coraco-arcuales) 

 is formed in Selachii from the ventral buds of a number 

 of myotomes behind the last gill-slit. To judge from the 

 number of ventral roots supplying it (cervical plexus, hypo- 

 glossus, cf. FuRBRlNGER, 1897, p. 404), the number of 

 myotomes contributing to the formation of this musculature 

 is greater in rays where some seven or eight ventral roots 

 participate in its innervation. In sharks this number is as 

 a rule from four to six of which none, one or two may be 

 intracranial, the rest being post-cranial. 



The secondary backward extension of the branchial basket 

 in Selachians is so strong that not only the cervical but 

 also the brachial plexus is pushed backwards in a curve 

 round behind the last gill-slit. As a consequence both unite 

 to a cervico- brachial plexus which distally divides again 

 into two branches, one to the hypobranchial musculature 

 and one to the pectoral fin. This common plexus was seen 

 by HOFFMANN (1901, p. 39) to form during development 

 in exactly the same way as is described by NEAL for the 

 hypoglossus of Petromyzon. In the sturgeon so great a 

 number of vertebral elements is assimilated by the cranium 

 that the whole brachial plexus, consisting of ventral and 



