ORIGIN AND STRUCTURE OF THE HEAD 81 



secondary gill-slits it could be imagined that their arrange- 

 ment did not correspond any longer to the mesomerism. 



Branchiomerism and mesomerism in lower Chordata. — Until 

 now we have left out of consideration the two lowest groups 

 of Chordates, Amphioxus and the Cyclostomes. A comparison 

 of these forms with the Gnathostomes, however, proves to 

 be decisive and truly fatal to FRORlEP's views. In Amphioxus, 

 as shown most clearly by WiLLEY's 1891) figures and by 

 HATSCHEK (1892), the arrangement of the gill-slits originally 

 corresponds to that of the somites, which from the foremost 

 to the last develop regular myotomes. Only secondarily the 

 backward extension of the branchial basket causes in this case 

 also the series of gilUslits to extend under myotomes origin- 

 ally situated behind them, while, after the "critical stage" 

 (WiLLEY, 1891, p. 202) has been passed, the number of gill- 

 slits, partly also by bipartition of the first formed, on the 

 contrary so much increases that it finally considerably 

 exceeds the number of myotomes of the branchial region. 

 Thus both the adherents to the view of GegENBAUR — VAN 

 Wyhe and those who follow FRORIEP can adduce the evidence 

 from Amphioxus as a support to their theory, the branchio- 

 merism here partly corresponding to the mesomerism and 

 partly being independent of it. There is no doubt, however, 

 that these conditions in the larva must be considered as 

 more primitive those that in the adult form. 



In Petromyzon, as demonstrated by Neal (1897, p. 447) 

 and KOLTZOFF (1901. p. 432), there is originally a complete 

 numerical (though not always strictly individual) corres- 

 pondence of the gill-slits and the somites. The eight 

 gill-pouches, of whi.h the foremost, the spiracular one, does 

 not break through, originate under the boundaries between 

 nine somites. These are here also well developed from 

 the foremost onwards, though only the post-otic ones, from 

 the glossopharyngeus-segment onwards, produce regular 

 myotomes, while from the pro-otic somites the eye-muscles 

 are derived. Again secondarily only does the backward 

 extension of the branchial basket cause myotomes of 

 postbranchial origin to be found above the gill-slits in 

 the adult. As NEAL has emphasized, a comparison of 

 embryos of 5 mm, and of 5 cm, shows that the dorsal 

 portion of the post-otic myotomes 7 — 12, which in the 

 earlier stage lay behind the last visceral cleft, in the 

 later stage lie anterior to this. Thus in the adult the 



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