268 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
quence there is found at the end of the larval period a “ critical stage ” 
of considerable duration, when Amphioxus possesses eight visceral clefts, 
which, if the homology above be correct, are exactly homologous with the 
eight morphological clefts of Heptanchus (Selachian) and Petromyzon 
(Cyclostome). ‘The evidence of the exact homology of the mouth and 
visceral clefts of Amphioxus. at its critical period with those of Craniota 
appears to me strongly confirmatory of the truth of the exact homology 
of segments in Amphioxus and Squalus as stated above. 
i. GENERAL CONCLUSIONS. 
The exact numerical correspondence of neuromeres (encephalomeres) 
and somites has been found not to be a purely accidental one, The 
ventral motor nerves (oculomotorius and trochlearis) of two successive 
encephalomeres, viz. II and III, are connected with two successive 
somites, viz. van Wijhe’s Ist and 2d, and the nerves VIT, IX, and X 
(Urvagus), by their topographic relations to successive somites 4, 5, and 
6, show a similar metameric correspondence between encephalomeres 
and somites. Where correspondence does not clearly exist to-day, as in 
the case of the abducens nerve, we have developmental evidence which 
suggests how such modifications may have taken place. 
Thirteen years ago Ahlborn (’84"), as a result of his examination of the 
evidence presented by van Wijhe (’82), stated it as his conclusion that 
in the head we have a dysmetameric neuromerism, which no longer 
repeats the metamerism of the mesomeres (somites), but is related to 
a series of other conditions dependent on both ectoderm and entoderm. 
Ahlborn likewise concluded that branchiomerism and mesomerism do 
not correspond. “Gegenbaur’s assumption, that the segmentation of the 
cranial nerves, related as they are to visceral arches, is comparable to 
the segmentation of the spinal nerves, which correspond with somites, 
still remains to be proved.” The evidence presented above certainly 
tends to make the assumed correspondence of mesomerism and branchi- 
omerism more probable, and thus indirectly to prove the homodynamy 
of the nerves which innervate mesomeres and branchiomeres. The re- 
cent evidence presented by Hatschek (’92), Kupffer (91, ’96), Price 
(96), and Miss Platt (797) from their studies on Amphioxus, Cyclo- 
stomes, and Amphibia points in the same direction, and thus favors 
Gegenbaur’s assumption. The comparative embryological evidence 
which has been given shows, however, that the adoption of Gegenbaur’s 
view by no means necessitates the assumptions later made by him (87), 
