268 bulletin: museum of comparative zoology. 



quence there is found at the end of the larval period a " critical statue " 

 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. 



^. General Coxclusioxs. 



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 1st and 2d, and the nerves VII, 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 ('!^2), 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 mesoraerism 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 pi'obable, and thus indirectly to prove the homodynamy 

 of the nerves which innervate mesomeres and bi'anchiomeres. The re- 

 cent evidence presented by Hatschek ('92), KupflFer ('91, '96), Price 

 ('96), and Miss Piatt ('97) 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), 



