Quarterly journal of microscopical science . y sac and relative shifting of the elementsol the neural crest and neural tube. These relations are shownin the embryo (6 mm.) reconstructed in Text-fig. 18. If it be granted that neuromeres have a primary segmentalvalue, then it may be said that there is one neuromere too manyoverlying the hyoid arch; but that such a segmental valueexists remains to be proved. To start with it rests on theassumption that the anterior head cavities represent a are present only in Galeus and Bqualus, bu1 in Amia, 472 G. RYLANDS DE BEER Reighard, and Phelp
Quarterly journal of microscopical science . y sac and relative shifting of the elementsol the neural crest and neural tube. These relations are shownin the embryo (6 mm.) reconstructed in Text-fig. 18. If it be granted that neuromeres have a primary segmentalvalue, then it may be said that there is one neuromere too manyoverlying the hyoid arch; but that such a segmental valueexists remains to be proved. To start with it rests on theassumption that the anterior head cavities represent a are present only in Galeus and Bqualus, bu1 in Amia, 472 G. RYLANDS DE BEER Reighard, and Phelps (16) have described the sucker as arisingfrom muscle anterior to the premandibular somite. Goodrich(7) has produced good evidence to show that the anterior headdiverticula of Amphioxus are homologous with the premandi-bular somites of Craniates, and it is more reasonable to agreewith Dohrn (3) that no segmental significance must be attachedto Piatts anterior head cavities. Then, supposing that the Text-fig. 13. Tr N4 F Ns Ne N7 Gl V Se Nj. Embryo 6 mm. long showing the relations of the neuromeres to theremaining segmented structures. subdivision of the third neuromere is not secondary butprimary and retarded (which it might be, for the third neuro-mere is just about twice as long as the following ones), it wouldbe necessary to postulate yet another gill-slit lost, to correspondto the extra neuromere. But perhaps the greatest objectionto tie segmental value of neuromeres lies in the fact that theyare altogether absent in Amphioxus, scarcely developed inPetromyzon, irregular and asymmetrical in Bdellostoma, andthai they ar« best developed in the higher craniates, birds,and mammals (Neal, 13). This strongly discountenances theirpaleogenetic value and suggests that they aie so I cannot believe thai the evidence from neuromeresfavours the assumption of a lost gill-slit. HEAD OF SQUALUS 473 The simplest explanation then, that originating from thework of Balfour, is t
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