104 THE ANCESTRY OF VERTEBRATES 



ventral side. Thus the whole hypoglossus is post-cranial 

 and begins far behind the skull. Now a comparison of embryos 

 of different lengths shows that the postotic myotomes-y— 12, 

 which are originally post-branchial, come to lie ultimately 

 above the hinder part of the branchial basket, i. e. become 

 epibranchial, by the backward extension of the branchial 

 basket. By the same process, as Neal was able to observe, 

 the ventral roots of these myotomes, passing originally free 

 behind the last gill-slit, unite one by one as the branchial 

 basket elongates and thus form the hypoglossus. The hypo- 

 branchial musculature in Peiromyzom shows a secondary (?) 

 segmentation corresponding to the situation of the gill-slits 

 but not to that of the epibranchial muscles to which myotomes 

 of post-branchial origin have been added (Fig. 20). 



As will be pointed out later, there is reason to consider 

 the Amphibians next after Petromyzon and before the 

 Selachians. We learn from the observations of Miss 

 Platt (1898, p. 452) that here ventral buds from three 

 myotomes, the last epibranchial (3'"'' post-otic somite) and 

 the first two post-branchial — 4*^ and 5*'' post-otic somites, 

 here at the same time the first and second post-cranial, 

 as we shall see afterwards — produce the hypobranchial 

 musculature. This is innervjited by a post-cranial hypc- 

 glossus-nerve composed of the ventral roots of the latter 

 two somites. 



in Selachians, as may be concluded from the statements 

 of Neal (1897, p. 450) and HOFFMANN (1898, p. 263), the 

 hypobranchial muscles are also produced exclusively or 

 nearly exclusively by post-branchial myotomes. Buds from 

 five myotomes representing in Acanthias, according to NEAL, 

 the 4**^ — 8**^ post-otic somite, form the hypobranchial 

 musculature. Since the fifth post-otic somite, according to 

 the conception of VAN Wyhe and his followers, is here the 

 first posf-branchial one, there is again only one epibran- 

 chial myotome that contributes to the formation of the hypo- 

 branchial musculature, while the others are post-branchial. 

 According to HOFFMANN'S (1898, p. 261, 263) statements 

 it is even exclusively from the post-branchial myotomes 

 that in Acanthias the hypoglossus musculature is produced, 

 that of the 4*'' post-otic somite not contiibuting to its 

 formation. This musculature according to NEAL is innervated 

 again by ventral roots belonging to the same myotomes 

 from which it has originated, viz. those corresponding to 



