267 
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 FRORIEP’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 gill-slits 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 
