. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. EMBHYOyiC DEVELOPMENT. 129 Strongly confirmed by the mode of development of the praemandibular cavities in the Cyclostomes. In these fishes, von Kupf- FER has shown that they actually appear in the form of a pair of diverticula from the anterior extremity of the archenteron (Fig. J2). If a comparison be made between Figs. 70 and 72, it will be at once manifest how close the correspondence is between the mode of de- velopment of the head-cavi- ties in Amphioxus and in Ammoccetes. In the Se- lachians the simil
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. EMBHYOyiC DEVELOPMENT. 129 Strongly confirmed by the mode of development of the praemandibular cavities in the Cyclostomes. In these fishes, von Kupf- FER has shown that they actually appear in the form of a pair of diverticula from the anterior extremity of the archenteron (Fig. J2). If a comparison be made between Figs. 70 and 72, it will be at once manifest how close the correspondence is between the mode of de- velopment of the head-cavi- ties in Amphioxus and in Ammoccetes. In the Se- lachians the similarity is hardly less striking.^ E)tdostylc and Pigment Fig. 72. — Horizontal projection of pharynx and praeoral endodermic exten- sion of a young Ammctirtes plaucri of 3^2 mm., reconstructed from a series of transverse sections. (After Kui'FFER.) Pra?oraI endodermic extension (prneorale Endodermtasche). pni. and ;;/. Praemandibular and mandibular portions of Iread-cavities. ph. Cavity of piiarynx. /, 2, J. First three pairs of gill-pouches. — Kupffer considers it probable that the mandibular as well as the pr:t- mandibular cavities arise from the single pair of endodermic diverticula. In the course of the following pages I have referred chiefly to the praemandibular In Fio" 64. there is to be cavities alone so as to avoid complica- tions. noticed a vertically placed structure lying in front of and contiguous with the club- shaped gland. It is a tract of very high cylindrical cells forming part of the right wall of the alimentary canal in homologised with the praemandibular cavity (see below, V.). Kupffer regards the praemandibular and mandibular head-cavities as rudimentary or meta- morphosed gill-pouches. This deduction is entirely foreign to the standpoint which I have adopted. The conclusion may seem plausible from the con- ditions observed in Acipenser alone; but when these are regarded from a comparative point of view, the deduction is. to my
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