270 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



followiug these may seem to favor Gegenbaur's view that the former be- 

 long to a palingenetic portion of the Vertebrate head which ended 

 with the 6th (van Wijhe's) somite (bounding neuromere VII posteriorly 

 and ventrally). The structural gap between the seventh and eighth 

 ueuromeres is not, however, so sharp that it should outweigh evi- 

 dences of similarity, and especially the evidence that somites 6 and 7 

 are indisputably serially homologous. I must confess that I cannot see 

 that the assumption of palingenetic and coenogenetic portions of the 

 Vertebrate head has added to the clearness of our morphological con- 

 ceptions, nor can I see that it is rendered necessai'y by any ontogenetic 

 or phylogenetic evidence now in our possession. Note, furthermore, 

 the disagreement of opinion as regards what is and what is not palin- 

 genetic or ccenogenetic among those who have been prominent as advo- 

 cates of this view, viz. Gegenbaur ('87), his pupil, Ftlrbringer ('97), and 

 Miss Piatt ('97). While Gegenbaur holds that van Wijhe's 6th somite 

 is palingenetic, Ftlrbringer regards the 6th, and possibly the 5th and 4th 

 somites, as coenogenetic. Miss Piatt, on the other hand, believes that 

 the 4th and 5th somites are palingenetic, but that the 6th somite is 

 probably coenogenetic. All this appears to me confusing and unneces- 

 sary. The terms coenogenetic and palingenetic are purely relative 

 terms. I hold the view that each metamere of the head may be re- 

 garded as coenogenetic in comparison with the metameres anterior to 

 it, the head gradually receiving accessions from the trunk. Gegenbaur's 

 famous " Kritik " of 1887 appears more an attempt to establish the 

 visceral arches as the essential criteria of cephalic metameres, than a 

 wholly unprejudiced effort to weigh the evidence both anatomical and 

 embryological which was at his command. I believe that the evi- 

 dence given in the present paper tends to strengthen the generally 

 accepted opinion, which Gegenbaur has sought to overthrow, that 

 the mesomeres in the head, like those in the trunk, afford the most 

 trustworthy criteria of metamerism. The dorsal (neuromeric and meso- 

 meric) segmentation must be regarded as more conservative than the 

 ventral (branchiomeric or splanchnic) segmentation. The lost elements 

 are chiefly the ventral ones. Their loss has indirectly caused the losses 

 in the dorsal elements, such as the disappearance of splanchnic motor 

 fibres from dorsal nerves and (?) of the thickening of the lateral zones of 

 encephalomeres I and II. 



It appears to me that the evidence now in our possession gives reason 

 to hope for an eventual solution of the head problem, not only as regards 

 the nature, but also the number of head segments. The problem, it is 



