140 



■tut ANCfiSTftY 6F VERtfeBftATES 



— -au. 



myotomes develops, those produced by the glossopharyngeus- 

 and vagus-somite differing externally in no way from the 

 next ones. The only indication of the beginning of the 

 process of reduction of the two anterior post-otic myotomes, 

 which we see atrophy in higher Craniates, is the dis- 

 appearance in the course of development of their deeper parts 

 (KOLTZOFF, 1902, p. 329). This 

 fact evidently is accounted for 

 by the circumstance that ihe 

 auditory capsule by its increaz- 

 irtg size no longer finds suf- 

 ficient room in one segment 

 but reaches (and the cranium 

 with it!) into the first and 

 second post otic segment, the 

 corresponding myocommata 

 being attached to it, as shown 

 by fig. 29, after GOODRICH 

 (1907, p. 40). According to 

 NEAL(1897, p. 446) the ventral 

 roots of the anterior post-otic 

 myotomes also get lost, these 

 myotomes being supplied by 

 branches from the ventral 

 roots of the 4*'' and 5**^ post-otic 

 myotomes (cf. fig 20). 



Just as in Ampfiioxus there is 

 a numerical, though not strictly 



som.2-^i 



m.i- 



individual, correspondence be- 

 tween the gill-slits and the 

 somites (NEAL, 1897, p. 447, 

 KOLTZOFF, 1902, p. 432), the 

 eight gill-pouches, of which the 

 first, the spiracular one, does 

 notbreak through, being situat- 

 ed originally between the first 



Fi?. 28 Reconstruction from a 

 series of horizontal sections 

 through the head of Ammocoetes 

 at the time of hatching, after 

 KOLTZOFF, 1902, p. 556. 

 au auditory vesicle, m.\ m" etc. 

 post-otic myotomes, so/n.^second 

 Somite, tr. ganglion of the trige- 

 minus, V. internal part of anterior 

 post-otic somites breaking down. 



and the ninth somite (thesecond 

 and the tenth of those authors who count the "praemandibular 

 somite" as the first). In front of the first somite a median 

 mouth is found which accordingly does not correspond to 

 that of i4mp/z/oxws (the left-sided gill-slit between the first and 

 second somite) but evidently to a pair of fused gill-slits in 

 front of the first or mandibular somite. 



